Porticus is a GUI for MacPorts, the open source command-line utility for installing command-line, X11 and Aqua software. Akin in many ways to PortAuthority, Porticus also provides Growl support, Sparkle updates, and an AppleScript library. It's snappy and light weight, and it's free.
Porticus allows description searches, auto-detection of outdated ports, MacPorts self-update, and port variant handling with variant selection before installing. If you've got MacPorts working on the command line, MacPorts provides a smooth way to handle finding and installing new ports. If you've shied away from the price of shareware MacPorts GUIs but are still craving a little Cocoa in your open source installations, give Porticus a try. Porticus requires an existing MacPorts install (1.4.x or greater), which can be downloaded from The MacPorts Project.