500 (and up to 7,500) free miles from Northwest Airlines

(expires July 31)

Northwest Airlines has a "Miles To Go" promotion to earn bonus frequent flier miles.  Most of the offers require a lot of work, credit checks, or signing up for new things, but one option, "Play the Miles To Go Sweepstakes and Instant Win Game," is a 1-minute, web-based game that is an easy way to pick up points in the game.  Play once and you earn 5 points worth 500 miles.  Play everyday and, according to the rules, "You'll earn five points for each daily entry."  Since the promotion expires on July 31, you get 61 days to play (i.e. 305 points), which will produce 7,500 miles for the diligent.