Gay marriage is now legal in Vermont, Maine, New Hampshire, Connecticut, and Massachusetts -- all of the New England states, except for Rhode Island. Gay marriage is also legal in Iowa.
New York State recognizes non-New York same sex marriages but does not have same sex marriage under its own laws (the link also has a handy map summarizing the current situation).
Broad domestic partnership laws that give partners all the state law legal rights of a married couple, but not the symbolic designation "married" are also flourishing, most recently in Nevada despite a veto by the Governor:
Nevada was one of nine states that amended its constitution to ban only same-sex "marriage" and not also "similar" unions. Three of those nine states (California, Oregon, and Nevada) now have broad domestic partnership laws.
California, of course, also has 18,000 couples whose same sex marriages remain legal and in force from the brief period last year after the California Supreme Court established a right to same sex marriage under the state constitution, and before the state constitution was amended by Proposition 8 which passed in 2008.
Broad domestic partnership law protections (the term "civil union" is used in some places) are also available in New Jersey and the District of Columbia. Limited domestic partnership law protections are available at the state level in Colorado, Hawaii and Maryland.
Many states have passed some sort of constitutional amendment or statute prohibiting same sex marriage, some of which also prohibit legal arrangements "similar" to marriage.
New Mexico is the only U.S. state which lacks anti-gay marriage statutes or constitutional provisions, and yet also lacks gay marriage or a domestic partnership law or court requirement.
Countries where same sex marriages can be entered into and recognized legally include: Belgium, Canada, Netherlands, Norway, South Africa, Spain and Sweden. Many countries have domestic partnerships or civil unions in some form, which varies considerably from country to country.