Apparently, 97 teams (82 of which are still currently league teams) have been involved in some point in the play offs (which started 30 years ago in 1987 - and, for two years, involved a relegation play-off with one club from a higher division battling it out with three clubs from the division below).
The handful of teams never to have sampled the pleasures of the play-offs are:
Arsenal, Everton, Liverpool, Man Utd, Spurs, Oxford, Coventry, Crawley, Newport and... reason for this post... the mighty Aston Villa.
Arsenal, Everton, Liverpool, Man Utd and Spurs haven't been relegated in this time (in fact, along with Chelsea - who lost one of those late-80s relegation play-offs, they are the six Premier League ever presents) and Coventry famously haven't finished in the top six of any division since 1970. I don't know much about the recent history of Oxford, Crawley and Newport.
Just did some research to see if any of the aforementioned clubs might drop off this particular list next year...
Newport may get relegated out of the fourth division this season and Crawley are only one place above them - so neither look terribly likely to be in the playoffs next year. Coventry just got relegated from the third division but are unlikely to bounce back due to the handicap of terrible owners (and, seeing as a passing train full of Coventry city fans once all spat out the windows at poor teenage me standing on a platform in my Villa shirt, I couldn't care less). Oxford Utd however finished 8th in the third division so look most likely to break their play-off hoodoo next year along with (depending upon how optimistic you are) our beloved Aston Villa.
Bringing back that relegation play-off might be fun...