1.) SteerMouse - tweak the mouse acceleration curve. If you're coming from a PC and you feel like your mouse doesn't move right, this will help.

2.) SizeUp - size and and position windows with hotkeys. For example, quickly set two windows to use exactly one-half of the screen each.

3.) Expandrive - mount S3 buckets, ftp sites, sftp sites as network drives. (Works great with textmate)

4.) Miro - excellent video player and torrent client with rss built in. Sort of like a torrent TIVO.

5.) Warp (http://www.ksuther.com/warp/) - adds some new methods for switching spaces. I have mine set to switch if I drag the mouse to a screen border while holding command.

6.) Also, if QuickSilver doesn't suit your tastes, give LaunchBar a try. Getting used to using one of those two apps pays huge dividends.