I am not a big fan of Safari, so I use Firefox as a browser. I don't know how Chrome runs on OSX but I guess just fine.
Adium for all your messenger needs. There are quite a bit of skins so shop around for those as well. I use Milk myself; gives it a nice clean and crisp look.
Steam of course. Chances are you already own games that are OSX compatible. I assume you bought one of those white unibody Macbooks and those run most Source games pretty well on low-medium settings.
You can of course 'acquire' a copy of Photoshop, but I found Gimp to run perfectly fine on OSX and that is officially free.
Only programs I downloaded were Adium, VLC and Chrome. And remember that any time you're in a public area, spam F3 (exposure or something) so you'll look cool. For some reason thats the only thing people do with a mac in the University. I always feel like going there and asking "you do know you can do more than just flip windows with that crap".
VLC is a necessity, as is Firefox or Chrome as Safari is pretty crap for web browsing, even if their template design for multiple open windows is nice.
I havent added anything else since I got mine.
Just got OpenOffice as it's free and just as good as Microsoft Office.
To be honest, if you google things like ''top 10 mac apps'' you can find numerous lists, often categorized by finance, office work, music etc etc.
Thunderbird is nice for emails too (love that program's icon).