In the episode titled "Lucky Stiff" that first aired February 7, 2011, Castle and Beckett went late-night undercover and things got a little steamy on the dance floor to "Loose" by Benny Blanco & Spank Rock from their EP Bangers & Cash. Earlier in the episode, Castle & Beckett tracked some suspects to a recording studio, where they were found rapping the soon-to-be-huge rhymes of "Get On The Floor"-- lyrics by Castle writer Alexi Hawley, and music by composer Jeff Russo (remember the band Tonic?). The opening song was "Money ($$$)" by Tim Myers (currently unreleased). I also like to call this episode "The One In Which Beckett Drove A Ferrari".
"Loose" by Benny Blanco & Spank Rock: iTunes :: Amazon