It's easy to find apps, but hard to find really good apps. One site that can help sort out the good from the mediocre is http://osx.iusethis.com/. You can list the apps you use and see what other people are using.
Sferik's list is pretty good. Quicksilver (above) is the app I miss the most whenever I'm using a Windows machine or someone else's Mac. It lets you launch programs from anywhere & do a whole lot more.
If you want apps that actually cost money, I like Apple's Pages (part of iWork) quite a bit for word processing. It's a lot sleeker than Word. I also like TextMate as a text editor. And Yojimbo (http://www.barebones.com/products/yojimbo/) is a great... thingy. I guess it's an information organizer. I put stuff in it so that I can find it later.