Ricky Williams
Credit: Larry French / Getty Images

Why he hung it up: After testing positive for marijuana for the second time, Dolphins running back Ricky Williams received a $650,000 fine and a four-game suspension. He opted to leave the game NFL.

Why he returned: Ricky's reasons for doing anything are unclear, but after an abysmal first season back in the league and a little bit of time in Canada, he settled down and became a viable NFL player again.

His Success: He continued to pile up yards, fail drug tests, and generally be one of the league's most interesting and enigmatic characters before retiring for good in 2012.