First up are the running backs, and SB Nation's Mark Sandritter sees former St. Louis Rams RB Steven Jackson as the top one who could be on the market, followed by former New York Giants RB Ahmad Bradshaw and New England Patriots Danny Woodhead. See the full list here.
SB Nation's Craig Powers lists Baltimore Ravens QB Joe Flacco as the #1 free agent quarterback on the market but as every Ravens fan knows, that will just never happen, be it through a new long-term deal or even the possibility to the Franchise Tag. However, Powers also likes former Washington Redskins, Oakland Raiders and Chicago Bears QB Jason Campbell as well as journeyman QB Matt Moore. See the full list of free agent QB's here.
There's a loaded list of free agent wide receivers and SB Nation's JP Starkey likes, in order, Patriots WR Wes Welker, Giants WR Victor Cruz and Kansas City Chiefs WR Dwayne Bowe, among many others. See his full listing here.
The cornerback position is a relatively thin one for free agency and SB Nation's Randy Booth see Dominique Rodgers-Cromartie as the best available, followed by most recently Patriots CB Aqib Talib and San Diego Chargers Quentin Jammer, who might have the best name for any cornerback. See his full list here.
Also, check out SB Nation's "Supergroup" of stories on NFL free agency.