One of the most remarkable performances in 2010 was Legarrette Blount's terrific season. He managed to rush for 1,007 yards in just 13 games, taking up just 201 carries for a terrific 5.0 yards per carry. Blount was one of the most elusive running backs in the open field, and was certainly the core of the Bucs' running game last season.
Still, I don't think that Blount is the kind of back you can build a running game around - not yet, anyway. For that, Legarrette Blount was too inconsistent last season. He would alternate 10-yard runs with stuffs, both caused by his tendency to ignore the blocking scheme and bounce plays outside. This was most evident in short-yardage, where he was woefully ineffective. I'm not the only one to hold that opinion, as Greg Cosell explained in detail why Legarrette Blount is not a consistent runner. He is still very valuable, and is great in the open field, but he needs more work to be a consistent runner.
Aside from Blount's qualities as a runner, he's also a liability in passing situations: he has not been able to make any impact as a receiver, and is too inexperienced to be a reliable blocking back. This makes the presence of Cadillac Williams on the team very valuable, but he is scheduled to be a free agent. Earnest Graham can take over those responsibilities, but Graham is not as productive as a pass-catcher, nor as tuned-in with the offense as a blocker. Kregg Lumpkin, the FB/RB hybrid who barely got on the field last season, is supposed to be a good pass-blocker and a capable receiver, but I've seen nothing of him myself.
As for complements to Legarrette Blount as runners, the Bucs relied on Cadillac Williams to provide that last season, and he did a good job. He was much more productive once Blount became the starter, and proved valuable as a change-of-pace back. Earnest Graham had one big run last season, for 61 yards, but was not productive as a runner otherwise in an season where he was limited by injury.
Kregg Lumpkin and rookie Allen Bradford are both good options as running backs, but they will likely have to fight for one roster spot because they're so similar. Both players are very big, one-cut, downhill running backs who will hit a hole hard and try to bull their way through. Unfortunately, neither player has done anything as an NFL player yet.
At fullback, the Bucs seem set. Earnest Graham has been a versatile player, who is a decent lead blocker. Similarly, the Bucs are trying to turn second-year player Erik Lorig into a versatile fullback as well. He was the starter when Earnest Graham went out with injury, and proved to be a good lead blocker. While he has not shown much productivity elsewhere, he has experience playing as a tight end in college and should be able to increase his role as he learns the offense.
In free agency, the Bucs are likely to re-sign Cadillac Williams to serve as the third-down back. If they do that, they won't bring in any other free agent running backs, as Earnest Graham and either Kregg Lumpkin or Allen Bradford will complement the running back stable. But there's also a chance they lose Cadillac, and they will have to bring in a running back to at least compete in training camp. As third-down backs, Jerious Norwood and Darren Sproles are the most prominent options and should both attract some attention from the Bucs. They could also go after Cadillac-clone Ronnie Brown, the Green Bay Packers' Brandon Jackson or even Reggie Bush, who is likely to be released due to his salary.