In 1999, my friend barely made the playoffs, and needed a WR for the week. So he picked up Q Ismail. He got him 6 rec for 258 and 3 TD's. He beat the best team in the league, who had R Moss and C Carter who both were SICK that year if I recall correctly. That is one of the best free agent pickups ever.
And one of the best fantasy performances not mentioned already.
How about J Stewart going for 5 TD's a few years back!! I didn't even have him in my lineup that week, mainly because Taylor was the starter. He didn't have any yards, but 5 TD's from a backup RB!!!!