Quicksilver

Quicksilver provides a LaunchBar-like launcher, an icon shelf, and a complete keyboard interface for most of Mac OS X, and has to be experienced to be believed.

• iCal plugin docs
• Abracadabra gesture plugin tutorial
• Constellation radial menu plugin docs

Mac OS X:

• LaunchBar

Windows

• AppRocket
• QuickStart
• Launchy

Linux / “Gnome”:Wikipedia:GNOME

• Gnome Do – very, very good.
• LaunchBox