Others mentioned here are great. In addition, check these out:

http://perian.org/ - adds native support to QuickTime for many video formats

http://www.fluidapp.com/ - Fluid, SSB (site specific browsers)

http://www.obdev.at/products/littlesnitch/ - LittleSnitch, Monitor and block outgoing internet connections

http://www.skitch.com/ - Skitch, screen capture and sharing

http://derailer.org/paparazzi/ - Paparazzi, Full screenshots of websites

http://www.sequelpro.com/ - SequelPro, MySQL gui (I like Querious, but that's a paid app)

http://wakaba.c3.cx/s/apps/unarchiver.html - The Unarchiver, unpacker program handles almost every format

http://freeverse.com/apps/app/?id=7013 - Think, helps you focus on a single app

http://www.heliumfoot.com/mercurymover/ - MercuryMover, resize, move windows with keystrokes (If you're OCD about window sizes like me)

Other honorable mentions: Coda, Transmit, TwoUp, Typinator, UnRarX, CSSEdit