Alright, you probably won't need as much applications as this dude from Flickr, but it's always useful to find out where to look for stuff.
In the list below, you'll find websites containing more Mac applications you can ever think of. This list is also to help myself remember where to find stuff to download.
1. Mac Specialist has a list that separates applications that are free and those that require purchasing.
2. Top 100 Mac Apps is a compiled list of Mac applications by a blogger. Great to see what you've missed.
3. Open Source Mac offers opensource Mac applications, of course. Tremendously useful for small businesses running on Macs. Think free and legal.
4. Cool OSX Apps is basically an applications review website that constantly updates with new stuff.
5. Versiontracker is the download.com version for Mac downloads.
6. Mac Tricks And Tips is another list of top 100 applications.
7. Apple also has their own database of interesting and cool stuff. Note that demo and shareware are mixed.
8. 10 Apps Every New Mac User Should Download. Out of the 10, I've 9 on the list. It's close to essential.
9. download.com - You should already know this.
10. del.icio.us - Better than Google in the way that find popular Mac application websites through related tags.
MacUpdate - Very similar to Versiontracker. Just in case you can't find what you want there.
FreeMacWare has a very comprehensive list of free stuff as well.