You may think you've never seen this guy beefore, but in fact you have- most notably in Old School, where he sang a memorable version of "Total Eclipse of the Heart." You may also remember him singing in Starsky and Hutch, and most recently in The Hangover.
In real life, Dan Finnerty is the fierce frontman of The Dan Band, an awesome group that remains strictly an LA institution as opposed to a household name as they deserve to be. The band consists of Dan and his two back up dancers/singers (pictured), and they tour around the country with a whole repertoire of songs made famous by female pop divas. The concept may sound flimsy, but with Dan's considerable vocal chops, well placed curse words ("Fuck no! I don't want no scrub."), hillarious banter, and chubby guy foxiness, they put on a damn good show.
Here's the Dan Band performing a Britney/Christina Aguilera/TLC medley featuring a dance involving hula hoops:
http://www.youtube.com/watch?v=5KYvpskat0I
I also heartily encourage everyone to youtube the Dan Band's version of Erykah Badu's Tyrone, Ring My Bell, You Oughta Know, and No More Drama. For this and so much more, he is the Hot Guy Of The Day.