MenuMeters (10.6 compatible version available soon; I've been using this for years – rock solid and low resource consumption)

Notational Velocity (best random note storage app ever)

XScreenSaver (a huge collection of screen savers; most I've deleted but there are some real gems like Substrate; I haven't tested with 10.6 yet)

Butler (configure custom menus, universal iTunes keyboard controls, and much more)

TextWrangler (formerly BBEdit Lite; the best free text editor for on a Mac)

Oh, and to play WMV files using QuickTime, get the free version of Flip4Mac.

It has Flip4Mac on it! I'll look into the other proggies.

I should have mentioned, it's used, so it's not running 10.6 or anything.

I can not survive without Textwrangler I use it every day. GET IT!

You'll like checking out VersionTracker, and see also my previous question about this, which has a link to an AskMe thread as well.

Adium for IM.

Hazel is freaking awesome for keeping your files tidy.

Dropbox Way better than MobileMe/iDisk, don't waste your money.

I prefer Picasa to iPhoto, which is kind of suck.

NeoOffice is OpenOffice for Macs.

Transmit for FTP.

I've not used Butler, but heard good things about it. Quicksilver's good too, but a lot of versions are unstable (as I found out when I tried to upgrade yesterday). Butler looks rather more complicated to me than QS, whose simplicity is a big part of the draw for me.