Mike Williams from the Bucs is one of the top guys to get in the late rounds. He's their #1 WR already and a good bet to make an immediate impact.
I think all three of the big rookie RBs offer good value (Best, Mathews, Spiller). That's especially true if your league gives points for catches.
Also keep an eye on Dexter McCluster and Montario Hardesty. I got Hardesty in the last round of my local league last night. He could be starting by midseason.
Some other players who might be in for breakout years:
QB Chad Henne, Dolphins
QB Matt Stafford, Lions
WR Johnny Knox, Bears
WR Mike Thomas, Jaguars
WR Pierre Garcon, Colts
WR Laurent Robinson, Rams