Will Smith -- Raps at Gabrielle Union's Birthday Party

Will Smith gave Gabrielle Union -- and the entire Internet -- the best present ever last night ... when he grabbed the mic at her 40th birthday party and rapped like it was 1991 all over again.

Smith, with the legendary Doug E. Fresh by his side, busted out a cool freestyle ... and then dropped "Summertime" AND the theme from "Fresh Prince of Bel Air."

Smith has caught the rapping bug lately -- remixing "Summertime" back in June and doing a little "Fresh Prince" on BBC's "The Graham Norton Show" in May.