One of the biggest general productivity boosters I've used is a good program launcher. I was a long-time user of SlickRun, and I've tried a few others. My current favorite is Executor - by far the best I've used.
Other options:
- Executor: My current favorite
- Vista Start Menu: Pretty good, actually, but Executor is similar (binds to Win+Z) and much more flexible.
- Quicksilver: For Macs only, but it seems to be the gold standard against which most other launchers are measured.
- Google Desktop: Press Ctrl+Ctrl and it's a quick launcher!
- AutoHotKey: Much,much more than just a launcher - more than I need, really.
- SlickRun: simple and unobtrusive
- Launchy: Seems to be the launcher of choice for many StackOverflow users :)
- Colibri: "Type Ahead - Information at the tip of your wings". Quite a cool concept.
- Many, many others. Scott Hanselman outlines some more here.
I realize that everyone will have their own preferences, but the question is: is there anything that really stands out in terms of speed, features, and especially productivity increase?