What tool makes you most productitive?
kunley: Emacs. Kudos to an univ professor who made jokes of anyone who was not using it. He used to say we'll be using emacs for decades if we really invest some time learning it. He was right.A REPL for any language used atm. Cool if coupled with the above.REPL/shell history with incremental search, like readline. This is the most powerful single feature I use. Hitting Ctrl-R and typing usually 2-3 keys gives me back the previously entered line I intended to find. If your REPL doesn't have it embedded, use rlwrap. Look at -H option to control where your history is saved exactly. Use big history size if you want to be reminded what you typed months ago - having 10/100k of history entries doesn't hurt really as it's still quick.Tab completion is cool as well. For rlwrap you can pregenerate keywords for completions (eg. your favorite libraries) and it will add completions for terms entered during current session.A good SCM for tracking changes even if working alone. I find centralized ones annoying; hopefully we've got Darcs & Git, both awesome.Using multiple desktops/workspaces to try new ideas in a space which doesn't clutter your current visual, so you don't waste your time rearranging it.Pen & paper. A habit of using it to get some distance. You can always scan a sheet of paper if needed. I know not every boss is going to accept such scan as a documentation ;), but preparing deliverable docs is something different than making useful notes to yourself.Lack of running IMs, MUAs, popup calendars and other disturbances during coding. All that stuff can wait. The focus is everything. 
One exception is when you do remote pair programming, then some VoIP app is handy. OTOH not many people work this way.A sign of a good tool is when you still enjoy using it after, say, ten years. Most of the above apply here.[edit: I meant MUAs not MTAs of course]