My favorite is definitely Commandos:Behind Enemy Lines. It included the 1st and the 3d/4th level of the game. The 1st level was pretty short and unremarkable, but the 2nd was a mind-boggingly awesome level with a huge concrete dam, which must have finished at least 10 times. I still remember it well: use the marine to swim to to the eastern compound, and fetch the rubber dinghy. Pick up the other commandos and have the spy collect a uniform in the compound. Take the spy to the main compound and switch off the electric fence. Have the sapper cut the fence and collect explosives inside the compound. Plant the explosives at the base of the dam and make your escape!

also had a remarkably fun demo in which you had loads of opportunities to play with the game's incredible fighting system. Unfortunately I read somewhere that the latter parts of the game feel rather rushed. Definitely would be a worthy addition though: [url=http://www.gog.com/wishlist/games/oni]http://www.gog.com/wishlist/games/oni

DUKE 3D

Yes!! The demo was the first two levels (from what I remember), so the first 10 minutes of the actual game felt like you were playing the demo again for the 100th time. But then you hit that 2nd nuclear sign, and you found yourself looking at the start of a brand new level you've never seen before! Good times...

edit: now I'm confused. According to [url=http://dukenukem.wikia.com/wiki/Duke_Nukem_3D_(Shareware)]this[/url], the demo included the entire first episode. Can't remember that all. What I remember is playing the 1st two levels over and over.

Shareware don't count hoser

Oni was a bomb ass demo.

Burnout Paradise Ultimate Box
Because it contains full game in it which can be unlocked if you can "manage" serial key somehow. ;)

I love you Canadians!

Too bad your beer sucks.

A few title that made me buy the game after playing the demo are: DreadOut, Overlord II, Mafia II, The Binding of Isaac.
DreadOut had a level that wasn't part of the main story, but it showed what you could expect for this game, and it did a fine job.
Overlord II has a pretty lengthy demo.
Mafia II had one level/chapter, and included the benchmark to see how well the game would run.
The Binding of Isaac demo is amazing with a lot of replay value.
Other than that, I remember playing with a friend on his console the demo for a Naruto game(don't remember exactly which title), it had two characters(Naruto and Kakashi), but we played it for weeks, many hours everyday, until he got the full game. I don't know why we did this.

As far as racing demos go, nothing can beat the time I spent with the NFS:Porsce Unleashed demo.
Driver had a pretty fun demo too, but I think it was timed. Come to think of it, so did Plane Crazy. I tried and tried to fly as fast as possible through the demo track with the naive hope that I might just manage to make it round a specific bend at the end of a valley, so that I can see a bit more of the track before the timer ran out. Man, it used to drive me nuts :P

Don't mind me I'm just sharpening my hockey stick.

Just Cause 2: just fun to replay over and over again.
Darksiders: It was generous as fuck. Around one hour (or more) of content.
Unreal Tournament: I had a lot of fun with those few maps.

According to Steam, I'm have clocked 7 hours in the Dying Light demo.

Guess I should just buy the sodding thing already.

Edit: Perhaps I should describe moar about the demo itself :p

So, you can play as much as you want, however the demo is limited to an hour per playthrough, can play offline or co-op, no saving progress.

It gives you an fairly sized sandbox ( a good chunk of the city for you to fool around if you want), one main mission, two side missions and some small random events and challenges. The one hour might not seem much at first but it gives you a fairly amount of time to upgrade your weapons, your skills and experience the night time feature (where you can get double XP at the cost of being chased by the more aggressive zombie-monsters things). At last, it supports some small mods, which is great.

Don't listen to him - Dieu du Ciel is really good! Peche Mortel is one of my favorites.

I wish I remember more about demos I've played. I think it was the XBox demo of Dragon Knight Saga that amazingly impressed me and caused me to grab the game years back, so that has to be up there (it was a game I didn't even know existed prior to that).

My favorite is The Killing Game Show, got it on a disc when I bought an Amiga magazine.

Euro Demo 42!!!