I love this: Reddit, Stumbleupon, Del.icio.us and Hacker News Algorithms Exposed!

Reddit's algorithm is probably my favorite, using logarithms to weight earlier votes more heavily. Although I do enjoy the StumbleUpon "safety variable" (which keeps it from being totally gamed). And the del.icio.us algorithm is kind of remarkable in its simplicity. As Danny says though, the "10,000 Pound Gorilla in the Room" is of course Digg's algorithm, which is stored in a single machine buried 5 miles below the surface of the Earth.