TurboGo
TurboGo is a Go playing program for the computer. It is especially written for inexperienced players or people who are not familiar with the rules of Go. For this reason, TurboGo has an extensive help section, containing the rules of Go, advice on how to play Go, an example of a game and a couple of Go problems, etc...
The homepage is http://www.turbogo.com/.
Author: Sbaguz
JanDeWit Another program I play against quite a lot is TurboGo, which is shareware, but only with the lowest 3 strength levels (out of five). This program has the advantage of being able to play on the three main board sizes, it's available in many languages, it can explain its moves, it has a 45000 move joseki database (I only use the 3-3 point invasion section for handicap games :-)
I think I'll stick with this program while I'm learning. The registration fee is quite modest, but I'm not sure what increase in playing strength it'll get you... (the program was European Computer Champion in 1999).
I love TurboGo! It's the best for someone learning the game.
Anonymous: I agree. I've now graduated from it, but it was my beloved partner when I was learning the game. I found it very user-friendly.