One thing I would suggest is signing up at
MacHeist. They constantly do deals and/or giveaways for random apps that I never would have encountered/considered otherwise.
Apple also has a
download page for lots of different apps -- some free, some not. These aren't all written by Apple; it's just a sort of conglomeration download page.
If you twitter, you might check out TweetDeck or Twitteriffic. If you blog, I'd suggest looking into MarsEdit. (If you use an LJ or LJ-clone site, I'd suggest xJournal). Adium is a good multi-client messaging system. ClamXav is a simple, free antivirus scanner. (Viruses aren't a huge problem for Macs, but there have been a couple in the last 6 months, so having something on hand isn't a bad idea.) Coconut Battery will help you keep tabs on your battery health and the number of cycles you've put it through. iStat Pro helps keep an eye on processes and processor use. If your system starts to slow or hang up or something, it'll help you identify where the memory hog is. If you use Gmail, you might want to download the Mac Notifier from Google. They have Picasa for google now, too. You can also get plug-ins for iPhoto to upload to Picasa and Flickr. Switch and iSquint are good for changing formats on videos and music files, especially when changing them to iPod formats. Ffmpg is good, too.
I'll keep brainstorming and see what else I can think of offhand. Enjoy! Let me know if you can't find some of the things I listed and I'll get you links.