The move of the Atlanta Thrashers to Winnipeg, Manitoba means I the defunct teams I have seen list. Over the years, I saw these now extinct teams:

NHL

Hartford Whalers
Quebec Nordiques
Atlanta Thrashers

MLB
Montréal Expos

I am pretty sure I saw the Winnipeg Jets, but my memory may be hazy. Of course, they *might* be reborn now, so it is a moot point.

I have also seen a few teams that changed their names in one way or another:

California Angels now Los Angeles Angels of Anaheim (I saw the California Angels, but never the Anaheim Angels or the Los Angeles Angels of Anaheim)
Washington Bullets now Washington Wizards (I saw them in both forms)

As for the Thrashers moving, the new Winnipeg team will stay in the Southeast division for a season which means 3 games in Manitoba for the Washington Capitals. I heard from someone claiming to be a Caps employee that they may be trying to have back-to-back games there to ease the travel burden.