Every division has players and coaches that pass between division rivals. Here in the NFC North, we have players like Brett Favre, Robert Ferguson, Darren Sharper and Ryan Longwell who went from Green Bay to Minnesota, like Rashied Davis (Bears to Lions) or Bernard Berrian (Bears to Vikings), and coaches like Mike Tice (Vikings to Bears). That's fairly normal. But what goes on in the AFC East is, to my knowledge, unsurpassed by any other division in the sheer number of cross-border connections.
Bill Belichick, the greatest head coach of his generation, was an assistant head coach with the Jets and, infamously, the Jets' head coach for exactly one day before he resigned and took the New England head coaching job. Bill Parcells coached the Patriots for three years, the Jets for three more (plus one as GM only) and spent another three with the Miami Dolphins. He hired Tony Sparano to be the Dolphins' head coach; after Sparano was fired in the 2011 offseason, he became the offensive coordinator for the Jets.
Meanwhile, Eric Mangini spent two years as a Jets assistant, five years as a Patriots assistant and three as the Jets' head coach. Chan Gailey spent two years as the Dolphins' OC and was just hired as the Bills' head coach. Pete Carroll was a Jets' assistant for four years, a head coach for one, and the Patriots' head coach for three. Brian Daboll, a Belichick assistant in New England, later became the Miami Dolphins' offensive coordinator. I know of no other division where so many coaches have bounced back and forth in this fashion.
That's nothing compared to the players that have swapped sides, however. The Wikipedia page on the Jets-Patriots Rivalry alone has a list of 63 players who have played for both teams from 1993-2008, including Ty Law, Vinny Testaverde, Shaun Ellis, LaMont Jordan, Danny Woodhead and Pro Football Hall of Famer Curtis Martin. Doug Flutie spent three years as a Patriot and three as a Bill before returning to the Patriots for the 2005 season. Drew Bledsoe, the first pick in the draft in 1993, spent eight years as a Patriot before being traded to Buffalo (2002-04).
The Dolphins traded Wes Welker to New England in 2007 in one of the all-time boneheaded NFL trades. Aaron Maybin, an ex-Bills DE, became an OLB for the Jets in 2011; Jason Taylor, an all-time great OLB, became a Jet in 2010 after spending most of his career with Miami. Thurman Thomas, another all-time great for the Bills, spent his last year with Miami. Chad Pennington was a Jet for eight years and a Dolphin for three. The late Junior Seau was a Dolphin for three years and a Patriot for four. Lawyer Milloy was a Patriot for seven years and a Bill for three.
This isn't just a Favre or a Longwell signing every so often, or bottom-of-the-roster players bouncing back and forth. This is head coaches and Hall of Famers, starting quarterbacks and Pro Bowlers moving among division rivals. And even more than free agent signings, these teams do business and trade with one another; the Parcells trade, the Belichick trade, the Welker trade and the Bledsoe trade are only the most prominent ones. I don't think any other division has been as incestuous as this one, at least in the last 15-20 years.