Your first stop should be open source mac, where you can load up on the essentials. I’d choose Firefox over Camino, because even though Camino is “more native” there isn’t a noticable speed increase or any shinier chrome. The next three on the open source page, Miro, Adium, are a must, and I find that Mac’s built-in Spotlight works well enough as an application launcher that I don’t need Quicksilver (although 99% of mac users swear by it). Add Handbrake, Burn, and Audacity and you can do just about anything.
Next up, I’d skip all the open source html editors, and get yourself a copy of Adobe CS3 or 4. To help with this, you can look for torrents in all the usual places, and a little app called iserialreader will assist in your serial activation. If you go this route for adobe or any other software, you’ll want little snitch, which is essentially an outbound firewall or whatever the proper term for such a thing is. Get version 2.0 or 2.0.1 because the newer version has a nagging feature. Use serials from iserialreader.
Because I’m the kind of guy who likes to see what’s going on underneath the hood, I install iStats from iSlayer, and activate the cpu, memory, and network montior in my menu bar (kind of like in Linux distros). This is a shot of what it looks like. That’s really it, because OSx is pretty cool. 
Yay!