Can you recommend one productivity-enhancing software application?

Ideally looking for third-party apps.

Don’t just say “Mac is better than Vista.” Nor need you mention any apps already bundled in with the OS (like Widgets or Task Manager). Can be either Mac- or PC-based(or both).

Valid examples include Launchy or Quicksilver.

Bonus points if you can explain exactly how it improved your workflow so the rest of us can duplicate your success.

[P.S.]: Specific widgets, plugins, or the like would be suitable.