The Mariners are now the only remaining existing franchise to never make it to the World Series.
That actually surprised me a bit when I heard it last night.
Seven current franchises have never won a WS: Rays, Rangers (third Washington Senators), Mariners, Nationals (Expos), Brewers (Seattle Pilots), Rockies and Padres.
By comparison, the other four major North American sports leagues (and especially the NHL and NBA) have had relatively larger numbers never play for or win titles:
NFL: 4 current franchises have never played in the Super Bowl (Browns, Texans, Titans and Lions) and 11 have never won (the four previously mentioned plus the Bills, Bengals, Titans, Chargers, Vikings, Panthers, Falcons and Cardinals). However, the Bills and Titans (the latter as the Houston Oilers) each won two AFL titles (and the Chargers won one), and the Lions and Cardinals each won two pre-Super Bowl NFL titles, so it's really only the Browns, Texans, Bengals, Vikings, Panthers and Falcons who've never won a title.
NBA: 7 current franchises have never played in the NBA finals (Hornets, Pelicans, Nuggets, Grizzlies, Thunder, Kings, Wolves and Clippers), and 12 have never won a title (the seven previously mentioned plus the Nets, Pacers, Magic, Suns and Jazz. However, the Pacers and Nets both won multiple ABA titles.
NHL: 4 current franchises have never played in the Stanley Cup Final (Blue Jackets, Jets/Thrashers, Wild and Coyotes/Jets), and 11 have never won the Cup (the four previously mentioned and the Sabres, Panthers, Senators, Predators, Knights, Canucks and Sharks).