Each NFL team is permitted to keep eight players on their Practice Squad. These guys are not part of the 53-man roster and make significantly less money over the course of the season (+/-$100,000) if they remain there all year. In addition, they are able to be plucked off the team by another at any point unless they are protected by being moved to the team's active roster.
This does not happen very often, but in 2005 the Cleveland Browns "stole" Baltimore Ravens QB Derek Anderson off of their Practice Squad and while he did not play that season, he played five games in 2006 before making the Pro Bowl as the Browns starter in 2007.
The National Football Post did a break-down of the Practice Squads in the AFC North, focusing on a player or two from each of the four division rivals. For the Baltimore Ravens, they like OLB Adrian Hamilton, a guy they think will be a contributor next season due to being "a quick and agile athlete with excellent pass rush skills."
OG Jack Cornell, who played a lot in preseason, is a big guy (6'6", 315 pounds), who according to the NFP story, "has the strength to tie up and eliminate his man from the play."
The story lists a few players on the Practice Squads of the Pittsburgh Steelers, Cincinnati Bengals and Cleveland Browns that might have a future in the NFL and perhaps even help shore up the Ravens defense.