Earlier this year, Rihanna called Ke$ha the "new It girl" in music, and now the pop sensations are joining forces on a 25-city tour! Ke$ha, who took the airwaves by storm with her hit singles "Tik Tok" and "Blah Blah Blah," will open for Rihanna's "Last Girl on Earth" North American tour, kicking off July 2 in Seattle. As if summer isn't hot enough! Tickets go on sale this Friday on LiveNation.com—check out Rihanna's official Web site for the full list of concert dates. In the meantime, catch RiRi perform on American Idol's results show tomorrow night.