Not too long ago, I wrote about Gnome Launch Box, a Quicksilver-like app on the Mac OS X, for Linux.
Whilst Gnome Launch Box is promising, the new Gnome DO seems to have improved on the features beyond what I have expected.
GNOME Do allows you to quickly search for many items present in your GNOME desktop environment (applications, Evolution contacts, Firefox bookmarks, files, artists and albums in Rhythmbox, Pidgin buddies, etc.) and perform commonly used actions on those items (Run, Open, Email, Chat, Play, etc.). GNOME Do is inspired by Quicksilver and GNOME Launch Box.
So what are you waiting for? Give it a whirl. :D