Usher has popped up on the remix to Omarion's Icebox. Prior to that there's been a handful of unreleased tracks from him ("Heavy", "Oooh Shawty", etc) and even a cameo in One Chance's "Look at Her" music video. Brian of Concrete Loop posted this snippet of a new Usher song called, "Make It Rain."
Usher - Make It Rain (snippet?) - download / alternate