These are in no particular order.
Portal:- Portal: Prelude This is the best mod EVER for Portal (according to ModDB's Mod Of The Year finalist list), at the time of this writing. As the developers say, it's not just a map pack - it's a total mod with a whole new story. Note that there is some swearing and innuendo, but nothing too bad.
- Quark This makes use of some interesting puzzles and game elements (walls that kill you, walls that physics objects cannot go through but you can) and even an outside part.
- Affinity This one has some nice BTS and testchamber scenes, and some very well-thought out puzzles. The ending is hilarious too. (I'm not about to give it away... try it yourself)
- Portal: Project Alpha This one should provide a good kick for those who, like me, watched the Portal trailer in its entirety multiple times. It is an attempt to recreate the Portal alpha, an attempt that they did pretty well at. There is also a Portal: Project Beta coming soon.
I haven't really tried mods for other games but if I find some good ones I'll share them with you. (I Hate Mountains for Left 4 Dead looks good)
