Who sings two and half men?

There's no one single person who "sings" Two and a Half Men. The show is a sitcom with a theme song, but it's not sung by a specific artist in the way that many other shows are.

The theme song, "Two and a Half Men", was composed by James Ingram and Bruce Sussman, but it's primarily instrumental. It's a very distinctive piece of music, though, and is instantly recognizable to anyone who's watched the show.