Off the top of my head:
Fallout 1 and 2- Along with PS:T below, form the holy triumvirate of computer RPGs. Really sad to see the "sequel" turned into a hybrid action game with stats and a dialogue tree such as Oblivion. We already had the excellent STALKER game out, this series deserved better IMO (yes I know, many out there love the new one).
Planescape: Torment- I love everything about this game except for the god awful "real time with pause!" combat that plagued Baldur's Gate and such as well. The rest of the game more than made up for it.
Operation Flashpoint- Not all that impressive upon release, but the mods made it incredible, and the single player campaign was done MUCH better (and much more fun) than the one in Armed Assault.
Close Combat 1-3- 2 being my favorite, I still replay these classic wargames, though not so much 1 anymore. The MatrixGames "CC: Cross of Iron" remake of CC3 has been fun as well.
Talonsoft's Campaign Series (West Front, East Front II, Rising Sun)- The games that got me into "real" PC wargaming, waiting for the MatrixGames remake of these (on that site as "John Tiller's Campaign Series) was like being a kid at Xmas all over again. Lots of depth, hard to master, but don't take a giant time commitment to learn.
The Operational Art of War- Another big one from Talonsoft and Norm Koger now re-released by MatrixGames as well, a huge, fun wargame with a powerful scenario editor, which ensures you never run out of things to play with it.
Europa Barbarorum- Rome: Total War mod that turned Rome: Total War from a dumbed down waste of $50 when compared to Medieval (1), into one of my favorite games. A history nerd's wet dream come true, when modded to work with the RTW "Alexander" expansion, the strategic AI gets a much needed boost as well.
Civilization 2- I played this one for a good 10 years until Civ 4 came out. Not sure why, but I just didn't like Civ 3 at all for some reason.
The Marathon Series- A little known series of shooters by Bungie that came out back in 1994 and beyond on the Mac, weren't released on the PC until no one cared about them any more. All the action of Doom 2 (which was huge at the time) with an engaging storyline similar to that of System Shock 2, these games were gems that are still relatively unknown.
Jagged Alliance 2- the best isometic tactical turn based shooter since X-Com.
HPS Simulation's "Squad Battles" series- another set relatively unknown outside of wargaming, probably my favorite squad level wargames on the planet. Again, relatively easy to learn, but not easy at all to master.
Medieval: Total War- the first one. IMO the best of the series, and the last one that the developers remembered to include a working AI for. I still play it with the "XL Mod" to this day, assuming it gets along with my PC.
Mircrosoft Flight Sim 2004- Have a load of add ons for it, and it doesn't require a supercomputer to run like Flight Sim X does- with my scenery add ons, FS2004 looks much better and runs much smoother on my PC than FSX did. A sim I can easily play all day.
A few more quick ones:
Galactic Civlizations 2 and add ons
The Gabriel Knight Games
The Monkey Island Series
No One Lives Forever 2
Falcon 4.0 and Allied Force
Space Rangers 2
Divine Divinity
Gates of Troy
- I tend to re-play a lot of my favorite games. Most of the new ones just seem like cookie-cutter re-releases of the same games that come out year after year with better graphics. I just don't get excited about them anymore like I used to.

I lean towards strategy games for most replayability, the jump in,jump out factor i guess. Once i start shooters or rpgs i develop serious OCD until i finish em again.
Civilisation 2- Probably the one constant on my system throughout countless upgrades, formats and rebuilds.
Medieval 2- Again just so much fun to startup, climb the tech tree as far as i can then start over again, often in the same day.
Homeworld 1 and 2- Not so much with vista issues lately but usually worth a couple installs 3 or so times a year.
And i will be shouted down here, but FF7 must go down as the most replayable game of all time eleven years and numerous poor imitations later.

Personally I've played Half Life and Half Life 2 through a bunch of times, as well as Fallout 1 (Didn't get into Fallout 2 quite so much).
Deus Ex for me had a huge amount of replayability, and every now and then I'll reinstall it and play though.
Oh, and when Metal Gear Solid came out years ago I played it relentlessly, back when the playstation was all the rage...

I've played through the original Legend of Zelda on the NES numerous times. These days, I play it with Zelda Classic.
Wipeout XL (2097)
Twisted Metal 2
Road Rash 2
Summoner 1 & 2
Grand Theft Auto 3
I grew up a console gamer, as you can probably tell. But these days, I'm more of a computer gamer, and interested in discovering all those oldies I missed out on.
Morrowind - my favorite game, ever. I have no idea how many hours I've played this over the years, but I would bet it's many thousands
Oblivion - I don't like this nearly as much as I do Morrowind, but even so, I've played about 800 hours
Pharaoh - one of the few games my first, crappy, PC could play, so I ended up playing it a lot, and I still fire it up every few weeks
Deus Ex - wow! I still haven't played anything else like it.
My interest in new games (or new to me) is not very high. I've played so many games so often in my life that a game has to be something special for it not to feel like a rehash of something I've already played. That said, I really am looking forward to some good old computer games. :)

How did I ever forget to list Deus Ex and System Shock 2? Both of those are classics, and still love playing them.

It's rather weak sequel.
The rather awesome (until the end-game action fest) Vampire: The Masquerade: Bloodlines.
Both are the closest matches you'll find.

Descent 1 and Descent 3. And Descent 3 Merc. And dozens of fan made missions.

Starcraft and Diablo II.
I can't begin to explain how much of my life I spent playing those two games.

The Blizzard stuff is quite high up the list of games that I almost always replay.
Diablo II and expansion
Warcraft III and expansion (I love the story)
Black Thorne
Then for non-Blizzard games:
UFO: Enemy Unknown
UFO: Terror from the Deep
Sacrifice
Dungeon Keeper Gold (every now and then)
Jagged Alliance 1 and 2
Final Fantasy 7 and 8 (mainly because those are the two I own for PC)
Mass Effect
There's probably bunches more, but I can never remember them all... too many games :P

Howdy.
Replaying, hmmm?
Red Alert I and II
C&C1 (not AS much, but on occasion.)
If anyone remembers it - Missionforce: Cyberstorm. (old Win3.x /Win32s title - and not the sequel. Actually one of the first things I'll put on a new system.)
Homeworld I and II
The original Diablo.
Archon (THERE's an old title.)
Airfix Dogfighter
European Air War
Myth I and II
If I could get them reliably working without an emulator (or even with) I'd add:
Wing Commander Armada
Subwar 2050

id agree myth has fair replay value but mostly thats because its a real bugger to kill Soulblighter without resorting to cheap clipping bug tatcics.
Also, ww2 flight sims lose there sheen after you've played IL2. It is quite simply perfect.

As far as I can see no-one has mentioned Vampire: Bloodlines.. Shame on ya'll
Nothing better than waking up and feeding on an innocent's Blood! and trying out the various disciplines and more challenging blood-lines (Nosferautu)..
other games overly-frequented:
Freelancer (modded), EvE (online), anything Square-Enix (anything!), Dues Ex 1 & 2 (our future?), Planescape Torment (bows to the greatest rpg), Morrowind (modded), Fallout games (classic +), Blade-Runner Adventure game (nostalgia), Puzzle Quest (Adhd), Legend of the Green Dragon (BBS remake of LoRD) :)

Quake.
I still enjoy playing Quake and its expansion packs using the glquake engine and hi-res textures. On modern hardware, it plays so smoothly and looks great.
Doom
Also a favorite in modern hardware and newer game engines. There seems to be no end to the number of custom levels available. My little nephews enjoy blasting Barney.
Carmageddon
The original is still the best for me. A insane racing game that never plays the same way twice, I still keep a Win98 box with a Voodoo video card to play the game in hi-res.

I have many favourites that I've replayed multiple times:
Baldur's Gate /TOSC-- about 12 times through
Baldur's Gate 2/SOA - ditto
Icewind Dale (the first one) - 7 or 8 times
Thief Gold & Thief 2 - probably 10 times each; add fan missions and it's about a thousand times
Gothic & Gothic 2/NOTR - 6 or 7 times each
Planescape Torment - 3 or 4 times
Deus Ex (the original game, not the sequel) - about 10 times
Might and Magic 7 - 6 times
Vampire Bloodlines (see -- someone did mention it after all) - 4 times
Splinter Cell (the first one) 4 times
Morrowind and Expansions- 3 times (at six months a play, that's a major time investment)
There are others, but of course it's the older games that I've replayed the most.

I've played Wing Commander: Privateer about 10 times. Also, Prince of Persia 2: the Shadow and the Flame