Tesuji Software Go Library
Thomas Thomsen: Thanks to Marco Scheurer (Sen:te) for hosting the library. As I also wrote on the computer-go mailing list, I consider this library (from the Go program Goliath) one of the best possible starting points for aspiring Go programmers. It is a shame it hasn't been online for a while, and it is really an awful shame it did not originally receive the attention it deserved when posted in 2001.
Among other things, the library contains an incredibly fast ladder-reading algorithm. In addition there are lots of gems about how to design fast data structures for Go (reusing/pooling arrays etc.) + loads of low level "secrets" (for instance, the trick of using a single dimension array with 20 elements per row for the board), etc.
On top of this, there is a loose ladder module, and an influence module. I haven't tried these modules, but Mark Boon considers the influence module THE most important one.
The library is in Java. It is well structured and documented, so it should be readable by non-Java programmers, too. I've benefited from many of the ideas in my own program (MadLab). So thanks to Mark Boon for generously making these sources available!
A newer version of this library from 2004 is available at Source-Forge: http://sourceforge.net/projects/tesujigolibrary The most notable addition compared to the old one is the addition of a pattern-matcher and some board-administration modules.