I watched Galaxy Quest again the other night. It's a hilarious spoof of Star Trek -- a group of washed-up actors from a cancelled science fiction TV series are mistaken for the real thing by aliens who have intercepted their TV broadcasts and who need their help to defeat their ruthless foes.
It's one of the funniest and on-target science fiction satires ever made. If you haven't seen it, I strongly recommend it. John Scalzi attempted something similar in his recent novel, Redshirts. It's not bad, but not nearly as clever as Galaxy Quest.