What Christian denominations (if any) permit gay marriage? Which churches have decided to permit gay marriage at a denominational level?
The United Church of Christ:


  Churches in the UCC can sanctify same-sex unions.[28] The resolution
  "In support of equal marriage rights for all", supported by an
  estimated 80% of the UCC's 2005 general synod delegates, made the
  United Church of Christ the first major Christian deliberative body in
  the U.S. to make a statement of support for "equal marriage rights for
  all people, regardless of gender," and is hitherto the largest
  Christian denominational entity in the U.S. supporting same-sex
  marriage.[29] This resolution (and the sanctification of same-sex
  marriages), however, is not supported by some congregations.[29] Of
  particular note, the Iglesia Evangelica Unida de Puerto Rico, a
  long-standing conference within the UCC, voted by a 3–1 margin to
  withdraw its affiliation with the UCC over the same-sex marriage
  issue.[30] And the Biblical Witness Fellowship, a conservative
  evangelical organization that is a small but vocal voice within the
  denomination, accepts only heterosexual marriage.[31]


http://en.wikipedia.org/wiki/United_Church_of_Christ#Same-sex_marriage