This is my overlay for Gentoo Prefix tested on OS X 10.6 (Snow Leopard). My standard overlay is here, along with some notes on what overlays are and how you use them. I won't go into much detail, because I'm not particularly interested in these packages, they're just a means to an end. Feel free to browse through the gitweb interface to see what's available.
Here's the layman XML describing the overlay.