A lot of different stuff, but the things I depend on the most for actual work and direct OSS offshoots:
Snow Leopard, Ubuntu server & VMs, XP VM (using VirtualBox)
Ruby, Haskell, Python, Javascript
TextMate, vim
git, gitx, git gui, vcprompt
bash
homebrew
postgresql, pgadmin
rvm
jquery
sass, compass
gemcutter, github
linode
chef
google apps
photoshop, illustrator
Firefox w/ firebug, firecookie, firequery, firerainbow, the web developer toolbar, cookie culler, user agent switcher, live http headers; webkit nightly dev tools; iPhone simulator from the iPhone SDK