Reggie McNeal was a speed demon. I was a freshman in '04 when he split the KSU safeties for the game clinching TD. It was just unfair. Darren Sproles had a lot of hype going into that game, but Reggie looked like the best athlete on the field that day. What sucks is that his speed worked against him later on. His senior year was sabotaged by Fran, making him run an option offense behind an average (at best) o-line. Then, the Bengals tried to make him a WR. Reggie was an incredible passer his junior year and should've been given a chance at QB. If he ran a 4.6 instead of a 4.3, he probably would've been given that chance.
Here's one I haven't heard mentioned yet: Danny Gorrer had blazing speed, and basically just that. The man couldn't tackle to save his life, but somehow hung on in the NFL for multiple seasons due to his speed.