It's as much about environment (meatspace) as environment (computer).

• A. Decent. Keyboard.
• A decent mouse
• My Leatherman
• Music (with volume and track control — nothing worse than poor, poor jazz interrupting your flow)
• Tea, because we don't have anywhere decent coffee
• vim
• screen
• (Physical) desk space
• CVS
• Firefox, and lots of extensions...
• Dead-tree reference material
• Enough water