20 different MLB teams have won the World Series in the past 30 years.  MLB has no cap, and thats 69% (29 years with 1 strike year not counting) of the time a "unique team" wins.  That's the highest percentage of the 4 major leagues sports.  Lets look at the last 30 years of all 4 major sports.

Looking at the chart, you can see the parity over the last 30 years.  You don't necessarily have to compare before and after cap to get valuable information, although it is interesting to see the natural curve show up, both pre and post cap.  The interesting thing here is that the NBA has had the cap the longest, yet the least amount of parity over the long haul.  If you look at the NFL you see parity both before and after the cap, just as Collin showed us.

In my own opinion, I believe the Parity in the NFL over the last 15 years has come from the evolution of the game, the college game, recruiting, nationalization of the sport, etc; not necessarily because of the salary cap.

I think the best stat to look at to determine parity for the NFL would be playoff teams before and after cap.  With only a one game series in the NFL playoffs, winning the Super Bowl is not the best determination of the best team, so it would be best to look at a larger field.  We could also look at the rank in the NFL of each teams player salaries, compared to number of wins, both before and after cap, for each season.  But frankly, that's a lot of research, and I really don't want to run the correlations!

C = capped year