Honestly, I would hold off installing a bunch of software until you have a need for it – or at least until you can appreciate the problem it solves.
OS X comes with a lot of tools out of the box, and I think it'd be better to stick with that stuff until you outgrow it or are no longer satisfied with it.
For example, people will recommend the VLC player, but QuickTime with the Perian plugin is what I use: more stable than VLC and plays all the formats. And it's one less app.
I didn't use Quicksilver for much more than a launcher when I had it. The Spotlight menu in OS X 10.5 is pretty much as fast as Quicksilver for launching apps. So maybe wait to install Quicksilver.
Same with Adium: iChat is good enough for me.
And people recommend Growl, but I found it more annoying than helpful. Pretty much every third party app registers itself, which means seeing tons of alerts I don't care about. (Unless you go into the Growl preference pane and unregister everything, which made Growl too high maintenance for me.)
The only essential things I'd recommend (again, only get them if they make sense to you):
TinkerTool http://www.bresink.com/osx/TinkerTool.html
– Gives easy access to hidden system preferences.
Perian http://perian.org/ –
A plugin for QuickTime that lets it play all the formats it doesn't play.
Paparazzi! http://www.derailer.org/paparazzi/ –
Makes taking webpage screenshots easy.
Service Scrubber http://www.manytricks.com/servicescrubber/ – Lets you prune the Services menu. (Don't worry about installing this until you see a need for it.)
TextWrangler http://www.barebones.com/products/textwrangler/ – Free text editor with terminal integration and excellent file search capabilities.
Transmission http://www.transmissionbt.com/ –
My favorite bittorrent client.