Mac OS X: Quicksilver has long been a favoured application launcher for Mac users who want ultra-customised keyboard control. While active development ceased for quite a few years, activity resumed last year and has continued apace, with version ß59 hitting servers last week.
Enhancements in the latest version include automatic application updates and the ability to open URLs in the background. One potential downer for owners of older Macs: support for PPC is being removed as of this version. You can find a full list of the updates in the change log.
Quicksilver is a free download for Mac OS X.