Matt Barrows of The Sacramento Bee has done some extensive research on which teams have held some form of player-led, informal workouts this off-season.
Barrows has a rundown by division highlighting who on each team is organizing the workouts, and how many players are attending. According to Barrows, players from 28 of 32 NFL teams have had some kind of informal workout so far.
The only teams whose players have not commenced workouts apparently are the Bears, Packers, Panthers and Seahawks. Both the Seahawks and Panthers appear to have workouts scheduled for next week.
Because information on these workouts is so scarce, it's hard to get a good feel for what is happening at these workouts. But from Barrows' list, it appears that there are three distinct forms of these player-led-activities. Small groups of around a dozen players who get together in loosely organized informal sessions, larger groups of between 20-30 players, often with both offensive and defensive players, and OTA-like affairs that draw around forty players.
Like the Cowboys workouts that are regularly attended by 40+ players, these OTA-like practices can feature work on plays from the playbook, the presence of external trainers and other signs of much more formalized and highly organized sessions. Along with the Cowboys, the Saints, Cardinals, Redskins, Falcons and Lions have organized such workouts. Barrows writes about these affairs:
One common denominator for teams that are holding more extensive practices: A strong leader - or two - who has organized the event and in some instances has paid for rank-and-file players to attend. Another factor: A state or a city, such as Atlanta or Phoenix, in which a lot of players spend their offseasons.
It remains to be seen what, if any, effect these different sessions will have once the season starts. But since I'm such a homer, it is perfectly clear that the highly organized sessions are of course the far superior option and all the other stuff is just kindergarten.