Mac OS X only: We've been fans of Quicksilver for years, but the universal application launcher and-then-some hasn't been in active development for a while. Alfred aims to pick up where Quicksilver left off.
That's good news for Mac users, though currently it's a pretty barebones alternative. It does application launching, supports quick web searches, and searches contacts as you type — features that are pretty old hat to QS die-hards. The plus side, however, is that the Alfred developers seem pretty serious about improving the app, adding new features, and most importantly, keeping the fruits of their labour free.
With this morning's announcement that Launchy is also available for Macs, it would seem Mac users have no shortage of options in the app-launching category. If you give Alfred a try, let's hear how you like it in the comments. Thanks gglanzani!