Gay marriage is legal in Belgium, Canada, Netherlands, South Africa, Spain, and the U.S. (Massachusetts).
Gays have the same rights as married heterosexuals, but only in civil unions or partnerships rather than marriage in Britain, Denmark, Finland, Iceland, New Zealand, Norway, Sweden, and the U.S. (California, Connecticut, New Jersey, and Vermont).
Gays have civil unions or partnerships with lesser rights than heterosexual marriage in Argentina (1 state), Czech Republic, France, Germany (3 states), Hong Kong, Ireland, Luxembourg, and the United States (Hawaii, Maine).
UPDATE (December 18, 2006): Stephen Frug has pointed out that even in U.S. states which have legal gay marriage or legal gay civil unions, they are still not equivalent to marriage, in part because of the U.S. federal Defense of Marriage Act (DOMA) signed into law by Bill Clinton. As a result of a provision in this law, the spouse of former Rep. Gerry Studds (D-MA), the first openly gay federal lawmaker, has been denied his pension benefits.
UPDATE (December 19, 2006): The December 9 issue of The Economist (p. 66) points out that the inclusion of Hong Kong on the list of countries with gay civil unions is a mistake. Hong Kong "is reviewing its laws in this area," but doesn't currently allow them.