OSX Specific: TextMate, Terminal, DTerm (awesome timesaver), Digital ColorMeter (Pixel measurement, CMD+SHIFT+C FTW)

General: Firebug, Dropbox (automatic backups!), Git, Photoshop, Customized .bashrc, .irbrc

Ruby specific: Sketches (with the default sketches dir symlinked into Dropbox)

These are probably 95% of where I spend my time. My workflow is generally: I use Spotlight and Finder to find things and then DTerm to interact with them or launch TextMate. If I want to dink around in Ruby I will go into irb and start a new sketch which will pop up TextMate and away I code.