Steam (if you have any Valve games), Adium, NewsFire, Chrome, "The Unarchiver", VLC Player.

I'm not really a gamer, so steam isn't really something I'm looking for. I'm definitely getting Adium, NewsFire sounds pretty cool too. If you're talking about Chrome the browser, I'm perfectly content with FireFox. And I already have VLC. Any more you can think of?

Movist (and MPlayer) is better than VLC for movies, especially if you have the right skin.

Bowtie and its hundreds of themes

Growl should be the first app installed on any new Mac, regardless of the user and its purpose.

Transmission for torrents.

iStat Menus and iStatPro. iStatMenus3 isn't free, but iStat 2 is if you can find it. I paid the $10, though. I've been using their apps for years and figured it was time I gave something back.

Unarchiver for sure.

Handbrake for all your converting and ripping needs.

Plex is the Media Center app of your dreams (much more customizable than Boxee and much more Mac friendly than XBMC which is what it is based on)

Perian and Flip4MacWMV are definitely needed. Allows you to play just about any video you want in QuickTime and online.

To be honest, most of the built in stuff is better than anything else you can download. There's a reason there is no real alternative to iTunes and why Mail reigns supreme. QuickTime X is more than enough to watch videos in if you install Perian. There is nothing better than Time Machine for backups. Preview is godly. TextEdit is pretty damn good if you don't need any advanced features.

Besides Growl, Perian, Adium, Photoshop and Firefox, every app I use is an Apple app (minus my customization apps, obviously)