FairVote. A non-profit educational organization located in Washington D.C. This is the leading organization in the United States dedicated to educating the public about alternative elections systems.
Californians for Electoral Reform. Much useful information about PR and especially on how to organize an effective campaign for PR in your community.
FairVote Canada. A site with a much useful information, including a good question and answer section. The organization aims to educate Canadians about the inherent inequities in their current voting system and the strengths and weaknesses of various alternative systems, such as proportional representation.
Electoral Reform Society Founded in England in 1884, the ERS is the longest continuous PR organization. This site contains particularly good information about the single transferable vote -- the Society's preferred form of PR. They also have an excellent analysis of the recent report issued by the Jenkins Commission that recommends a new voting system for British parliamentary elections.
Proportional Representation Society of Australia Like the ERS, this organization is a long established proponent of PR, especially the single transferable vote (called "Hare Clark quota preferential voting").