The top 8 North American sports leagues to challenge an established professional league
#1--American League of Professional Baseball
  They fought the good fight, playing for nearly 100 years as a separate but equal league.  They were not forced to merge with the National League until 2000, at which time they merged on equal terms into a completely new organization, Major League Baseball.  They only had one franchise, the Milwaukee Brewers, leave the league in over 100 years of operation.  Milwaukee abondoned  the American League for the perceived greener pastures of the National League in 1998.  Until 1998 the American League was the ONLY league in American professional sports to have never lost a franchise.
#2-- American Football League
  Started as a challenge to the National Football League, the AFL became a league of nearly equal status before it joined the NFL in 1970.  Even before joining the NFL, the AFL had a national television contract, and for 4 years its champion played the champion of the NFL for football supremency in the Super Bowl before the merger.  Even before the AFL joined the NFL, they won 2 of 4 Super Bowls with the New York Jets and Kansas City Chiefs taking home titles over the established NFL.  Even more impressive than the ALPB, the AFL never had a franchise fold and ALL of its franchises merged on equal terms into the NFL.
#3--American Association of Professional Baseball
  In the 1800s, the AA was the equivilent of the American League in the 1900s.  They consistantly challenged the National League for baseball supremency.  This league lasted for 10 years before folding.  Throughout its history, 8 clubs from the American Association began to play in the National League.  Cincinnati even left the NL to join the AA before rejoining the NL.  This franchise still exists as the Cincinnati Reds in MLB.  The Pittsburgh Pirates, Los Angeles Dodgers, and St. Louis Cardinals all got their start in the American Association.
#4--American Basketball Association
  The ABA was developed to challenge the NBA and to provide an opportunity for the players to receive more money.  Throughout the league’s 9 years of existance there were only 12 total franchises, although many franchises moved around from city to city.  Sensing that the league was hurting the NBA, the NBA offered 4 of the franchises the chance to join the NBA.  The San Antonio Spurs, New Jersey Nets, Indiana Pacers, and Denver Nuggets all got their start as ABA franchises.  San Antonio became the first franchise to win an NBA Championship in 1999.  2003 saw the first all-ABA finals as San Antonio won their second title over the New Jersey Nets.
#5--World Hockey Association
  The WHA became famous when they began to raid the NHL of many of its top players with large contracts, including an unheard of $1 million for Bobby Hull from the Winnipeg Jets.  Over the course of 7 seasons, teams and franchises came and went, however there were some strong franchises.  In 1979 the NHL invited the Quebec Nordiques, Winnipeg Jets, New England Whalers, and the Edmonton Oilers to join the league.  This effectively ended the WHA.  Unfortunatly these clubs were all located in small markets and eventually three of the franchises would move.  The Edmonton Oilers, however, have become one of the greatest franchises in all of sports, winning the Stanley Cup 5 times while in the NHL.
#6--National Basketball Association
  Few know that the NBA began as a rival to the established National Basketball League. The NBL, however was in decline when the NBA began operations, thus the NBA's success was not unexpected.  In only two years, the NBA, with backing from many NHL owners, knocked the NBL out of business and allowed the 9 strongest franchises to join the NBA.  This decision of the NBA would prove to be fortouitus as the franchises that they accepted from the NBL included teams that would become the Los Angeles Lakers, Detroit Pistons, Atlanta Hawks, and Sacramento Kings.
#7--All American Football Conference
  Although it only lasted for 4 years, the AAFC provided a lot of competition for the NFL and when three of its teams joined the NFL, they would end up being some of the most successful franchises in NFL history.  The Cleveland Browns dominated the AAFC, winning the title each year, and when they joined the NFL in 1950, they continued where they left off, winning the NFL title in their first year.  The Baltimore Colts and San Francisco 49ers also joined the NFL from the AAFC.  These three clubs have combined for 12 NFL/Super Bowl Championships.
#8--Women's National Basketball Association
  Founded with the backing of the NBA, this league sought to challenge the young American Basketball League.  Although the ABL had, in the opinion of most, a superior product, higher player salaries and better players, the WNBA was able to use the power of the NBA to bully television networks NBC and ESPN into broadcasting games.  This effectively ended any chance the ABL had of succeeding.  Today the WNBA is still alive, but floundering as the NBA has pulled much of its support of the league.