Currently a free agent: Corey Hilliard (I believe he is a restricted free agent)
New to the team: Johnny Culbreath (he was picked by the Lions in the seventh round of the 2011 NFL Draft)
The starting spots at offensive tackle for the Detroit Lions aren't all that interesting. Barring an injury, Jeff Backus will be Detroit's starting left tackle for the 11th season in a row. And barring Gosder Cherilus not being able to recover from offseason knee surgery, he will be the starter at right tackle.
The interesting thing to watch will be the battle between the backups for roster spots. Corey Hilliard played his way onto the 53-man roster during last year's preseason, and I would expect him to return as a backup. I would also expect Jason Fox, who is finally healthy, to return and continue his development. That could leave Tony Ugoh on the outside looking in if the Lions only keep four tackles, but, of course, things can always change with injuries.
As for Johnny Culbreath, the Lions' seventh-round draft pick, he is a prime candidate for the practice squad. He is a player considered to have a lot of upside, but he is very raw and will probably take some time to develop.
Need going forward?
Like most positions on offense for the Lions, they seem to be set at offensive tackle. I could see an undrafted free agent being brought in as a camp body, but unless the injury bug bites Detroit's offensive tackles, this position is in pretty good shape depth-wise.