Introduction to Pesapallo [more]
Features rules, diagrams, history, comparison to baseball, and teams in Finland.
http://www.nic.funet.fi/index/sports/pesapallo/pesapallo.introduction.ver102

Pesäpallo [more]
Includes a history of the game, plus the official rules and vocabulary.
http://www.pesis.fi/nuorisopesis/international_site/

Wikipedia: Finnish Baseball [more]
Brief article describing the sport and its history.
http://en.wikipedia.org/wiki/Finnish_baseball