CoverSutra - very nice iTunes controller. it stays out of the way and works completely off of hotkeys.

Xtorrent - just went final. very nice bittorrent client. Just search for what you want and you'll let a list of items... all you have to do is click download. It'll also search Google and Yahoo in the background as a safe measure.

Shapeshifter - changes the GUI.

Silk - Changes system fonts

Flock - a web 2.0 version of Firefox. There's a very nice Flickr uploader included... which is the main reason I use this.

TextExpander - a small productivity tool that'll take a keyword and make a sentence off of it based on your settings . Not so much that I'd use it for it's initial use, but, someone created a plugin that gives you all of Microsoft Word's list of mispelled words and fixes, so changes come on the fly.