UPDATE: As these things can evolve quite rapidly, here’s an updated version of the US map, dated January 2010:
This map takes into account the California repeal of 2008 (California is still partially colored in purple in the first map above), as well as the granting of marriage rights in Iowa in 2009 and in some East coast states.
Here’s another version, comparing the degrees of restrictions on the rights of same sex couples in the different states of the U.S. (there’s a lot of variation between the extremes of a total refusal to grant couples any sort of recognition and a legal right to marry):
(source, there’s an interactive version here showing the evolution since 2000, the major milestones and the detailed provisions in each state; another interactive map is here)
UPDATE: and this is the situation dd. Februari 2012:
This is a very detailed map:
(source, click image to enlarge)
UPDATE: during the Presidential election today, November 7th 2012, Maine and Maryland also voted on ballot initiatives regarding same-sex marriage. Majorities of voters in both states approved it.
UPDATE: here’s an animated map for the years up to 2013: