For the people who are curious about where to start with Doom modding, this thread could be a place for people to put their suggestions. Most of these will probably require a sourceport of some kind, I personally like GZDoom (hardware renderer), though regular ZDoom (software renderer), Zandronum (multiplayer) and Chocolate Doom (as vanilla as possible) are out there too.

For example, I like [url=http://realm667.com/index.php/en/component/content/article?id=289:the-ultimate-torment-a-torture]The Ultimate Torment and Torture,[/url] it's heavy on the gothic industrial feel, almost like Quake. It's difficult, but not the insane difficulty of several other mods.

Back to Saturn X is also a nice, high-quality mod. Very nice style.

Epic 2 is one of those insane difficulty mods I mentioned up there, but it's got great style and it is absolutely sublime with the gameplay mod Wild Weasel's Nazis! which basically turns it into a very solid Wolfenstein-tye game.

And speaking of gameplay mods, I would be remiss to not mention the great Demonsteele which turns Doomguy into a tiny anime lady with a sword. Fast paced and very '90's, it's a love letter to heavy metal music and silly anime.

This is by no means an exhaustive list, after all, there's been over 20 years of Doom modding, so please add to this list!

The only one I remember playing much of back in the day was Aliens TC. Awesome mod.

http://www.moddb.com/mods/aliens-tc

even if it's in beta stage: http://forum.zdoom.org/viewtopic.php?f=19&t=47574 Doom: The Golden Souls 2 for GZdoom

If you want to play with vanilla doom you should try Plutonia 2 (an unofficial sequel to Plutonia Experiment): http://www.rabotik.nl/pluto2.html (you must have the original plutonia WAD)

by the way: stay away from terry-wads!

Here's one of my favorites. :)

There's way too much good stuff the Doom community has made. If you want the best they have to offer, check out the Top 100 Doom WADs and the Cacowards.

As for personal recommendations, here's a few of my favorite gameplay mods:

DemonSteele (Doom meets Shadow Warrior)
Guncaster (play as a gun-wielding dragon sorcerer)
Hideous Destructor (Sci-fi Dark Souls. For bad enough dudes only)
Russian Overkill and Trailblazer (very fun weapon mods)
Reelism (Doom meets Smash TV)
Smooth Doom (adds some nice visual enhancements but keeps gameplay intact)

If you'd like to check out more, visit the Projects page on the ZDoom forums. There's plenty of stuff to check out.

Batman Doom - Not only do you get to play as the Dark Knight in Doom, but it's also one of the more technically impressive mods of its time. There is a patch for it that works on ZDoom (it will not work on dev builds though).
Unloved - Image Doom in the realm of Silent Hill.
PlayStation Doom Total Conversion - Play the PlayStation versions of Doom on your PC without emulatiors plus all the enhancements GZDoom offers. I also made a guide for setting this up this on Steam.
Action Doom and Action Doom 2: Urban Brawl - The former basically turns Doom into a first-person Contra-styled game whereas the latter puts you in a Sin City-inspired brawler with pseudo cel-shaded graphics. Both of these are freeware, but fair warning, Action Doom 2 does have some NSFW content.
Harmony - An old-school styled total conversion that harkens back to first-person shooters like Duke Nukem 3D and Doom. Also freeware.
Mega Man 8-bit Deathmatch - Play as the Blue Bomber as well as many of the Robot Masters in this total conversion that turns the classic 2D Mega Man games into a deathmatch-styled FPS game. Originally required the Doom II WAD, it now uses own game files to play so it's completely freeware.
Stronghold: On the Edge of Chaos - A Doom mod with tower defense-styled gameplay.

If you love gorgeous visuals and absurd difficulty, then I really recommend . And well, anything else from its authors -- Ribbiks notably made [url=http://doomwiki.org/wiki/Stardate_20X6]Stardate 20X6 and Swim With The Whales which are undeniably artistic and frustratingly hard. Danne made the Combat Shock series which are also impressive, especially the second. In the same vein, take a look at Sunder. They all excel at conveying huge, surreal, demented architecture in Doom, creating scenes that are both beautiful and oppressive.

Speaking of artistic wads, and getting less absurdly hard, I strongly recommend trying out BAUHAUS. It's short, it's sweet, and it's unique.

While I am recommending stuff left and right, everyone needs to experience the insanity of Going Down. It's made by Cyriak Harris who is somewhat famous for making videos where random things spiral out of control and turn into nightmarish abominations, and Going Down is a faithful transposition of that idea to video game level design.

Finally, I'd suggest giving ZDCMP2 a try because it's currently the single largest map ever made and it's a very impressive effort. Be wary that you need a powerful computer to run it (it has thousands of textures and several hundred actors) and that it can easily take about three hours to complete especially if you're hunting down each of the 26 secrets.

The two Memento Mori MegaWADs are excellent, and probably the best of the old (pre 1997) WADs.
Nothing groundbreaking, but solid, high quality WADs in the Doom 2 style. Certainly better than some commercial half-official MegaWADs that were published at around the same time, like Master Levels and Perdition.

I have not gotten past any of the actual levels of a Ribbiks map because they're so tough, but I love Stardate 20X6 and Swim with the Whales, they look so fantastic. Also, Going Down is pretty tough, but also great. Things really start going off the rails in the level Time Warp, which is where I am currently.

I will also toss in Scythe 2. It starts off pretty simple but it gets insane later on. I've heard the other two, and [url=http://doomwiki.org/wiki/Scythe_X]Scythe X are good too, but I haven't gotten to them yet.

ZPack is a community project that gets some love, and what little I've played is pretty good, though E1M3 is a timed level that I honestly hate. If you do too, skip it, I need to get back to playing it at some point.

In the same vein as Aliens TC brought up before, there's Zombies TC based on the classic Dawn of the Dead. There's a version out there that's packed up to work perfectly with (G)ZDoom, I'll try to find a link in a moment.

Also, there's ZanZan made by ACE Team, makers of Batman Doom and Zeno Clash. It's a weird, weird mod, and rather tough, but absolutely fascinating.

Then, if randomising monsters and weapons and shit is your thing, there's Aeons of Death. It got Worst WAD in the 17th Cacowards by someone who obviously hates fun (ok, not really, it's just not for everyone). It randomises all the enemies and weapons to include just about everything. And I mean that, Heretic, Hexen, Quake, Redneck Rampage, Blood... weapons and enemies from these and many, many more games are in and it makes for a wild and crazy, crazy experience.

If that sounds almost good, but you want something a little more restrained, there's scalliano's 667 Shuffle which uses stuff from the Realm 667 Bestiary and Weapons to make a unique experience that fits a little more into the traditional Doom look.

Don't forget Eternal Doom, by team TNT : http://www.teamtnt.com/ixet.htm.
A very hard 32 levels WAD, and combined with Brutal Doom, even harder.

If you liked the original Doom levels (or just want something that feels like a nostalgia trip yet completely new at the same time), I would STRONGLY recommend Doom the Way id Did, and Doom 2 the Way id Did:

http://www.doomworld.com/idgames/levels/doom/megawads/dtwid.zip
http://www.doomworld.com/idgames/levels/doom2/megawads/d2twid.zip

As the name imples and description says, these are built very much in the style of the original levels. But at the same time, they feel a little more polished and advaced compared to the original levels (some of which are, to be honest, not really that good).

Perhaps they're something that id would have made if they had had some more time to try things out and get experience with doom mapping. Remember, they were beginners back then, just like everyone else. The first person shooter genre practically speaking didn't exist back then -- wolf3d with its hitscanners and flat, tile based layout made of orthogonal walls doesn't come close to the finesse of the architecture and gameplay allowed by the Doom engine.

I think Ultimate Doom the Way id Did is also in the works... (check out the doomworld forums)