Seule said:First, get Of Sound Mind. It's the best module I've ever read, and my players liked it too, when they weren't running in fear.
Second, the Freeport Trilogy is very good. I'm not sure if they are all still in print (Death in Freeport, Terror in Freeport, and Madness in Freeport. Hell in Freeport is, IMHO, not nearly as good). Plus, the fact that they all tie together can really throw your players for a loop, if they don't expect things to tie together so closely.
If Thoughts Could Kill is great if you have psionics in your game. If not, skip it.
The Adventure Path modules have several quite good ones, but some are not so good. Read reviews before buying.
I had a lot of fun with What's That Smell? by John Wick. It's also very customizeable.
Finally, get a subscription to Dungeon magazine. You won't regret it, I find 1-2 or more useable adventures in every issue.
--Seule