This is going to be unprompted, quick, full of generalizations, and it's about something that's more of a running joke than a real concern. But I still feel like saying it.
All free agents in the NFL are players that were not brought back by their team, and understanding why that was so is important.
Good teams are better talent evaluators than bad teams. Good teams get more out of their players than bad teams. Good teams are better at hanging on to good players and letting unessential players leave.
Two of the Seahawks' players that helped them win a Super Bowl were Michael Bennett and Cliff Avril, signed from the 7-9 Buccaneers and 4-12 Lions. Bennett was let go because Greg Schiano is an ego-mad prick, and Avril was let go because the role he would have been paid for (every-down end) was incongruous with his real talents (rushing, rushing, rushing). In Seattle they thrived.
What Mayhew has been great at is snatching players from bad situations and getting better value out of them for the Lions. Chris Houston, Rob Sims, Corey Williams, Shaun Hill, Mike Thomas (OK, pobody's nerfect). And there aren't many teams that have been more incompetent over the past few years than the Jacksonville Jaguars. If you're looking for misused players that can shine in a better system, don't go to the playoff teams, go to the crappy teams. That's why kicking the tires on an ex-Jaguar is fine by me.