With the Nextel 7520 the following have been tested to work:
Motorola HS810, 820, and 850
Plantronics 2500
Jabra BT250
And a couple more, cant remember...

And whatever you do, don't get one of the cheap Bodyglove ones...worked with someone for an hour Monday on pairing one and no luck whatsoever...

Oh, and they will interact with some bluetooth-enabled Lexus and BMW automobiles...but don't worry about interference unless your car starts veering to the right each time you direct connect O_o... Just kiddin.