The song is "Bra" by the 70's funk band Cymande (pronounced Sah-mahn-day). If you think you recognize it you are either are De La Soul fan (the song was sampled in De La Soul's "Change in Speak" from the 3 Feet High & Rising Album) or you remember it from the club scene in the Spikke Lee joint 25th Hour when a young and severely fucked up Anna Paquin is trying to seduce a not-so-young (or hip) Philip Seymour Hoffman (Paquin's high school teacher in the film) at Monty's going away party at the Russian DUMBO club. For my money the second best scene of the film (clearly the mirror scene is the best scene for its pure balls) , especially the part when Hoffman leaves the bathroom after cheddarballing with Paquin and realizing he fucked up. Great scene, great song, great Hoffman. Not much more you need in a film for my money.