Houston Texans has never made the NFL Playoffs.
With the Tampa Bay Rays making the playoffs in the 2008 season, all MLB teams have made the playoffs at least one time.
In MLB, no. With the Tampa Bay Rays making the playoffs in the 2008 season, all 30 MLB teams have made the playoffs at least once.
Washington Nationals. don't know any others
The Houston Texans 2012 answer: all NFL teams made it to the playoffs.
Every NHL team has made the playoffs at least once. Atlanta and Columbus have only made it once.
all teams made the playoffs once
Arizona Cardinals Chicago Cubs
All teams in the NHL have made the playoffs. However, the Atlanta Thrashers and the Columbus Blue Jackets have only accomplished this once in their short time in the league.
Now that Tampa has made the playoffs, every single MLB team has made the playoffs at some point in its history, although Washington hasn't made the playoffs since they moved to DC; their last playoff appearance was in 1981 as the Montreal Expos.