Following a conversation last night…
Star Trek fans are Trekkies (or even Trekkers).
What about other shows that have loyal and ultra-loyal fans?
Buffy seems to be immensely popular atm. What do we call them? My Slayer-infatuated housemate thinks Buffins though I think Buffoons may be better. 
Friends - Friendlies?
The Practice - Pracccas? Plan Beans?
What other shows could have cool and/or interesting names applied to their worshippers?
the natural choice for an X-File fan is obviously X-Phile.
+s on X-File, of course.
Only specific ones I’ve ever heard of are Trekkies and X-Philes. For the rest, we just call ourselves fen. 
I know many of the Buffy fans (who me? ;)) simply call themselves Buffy-Watchers (play on roles). Seems like each character has their own “sect” of fans, who call themselves something special. I know there are GASPers and Headcases (Anthony Head aka Giles). I’ve also seen Buffonian websites (based on a joke the character Xander once made)…
I would imagine that fans of “Angel” could be Angelites. Dawson’s Creek, D’Creekers?
BTW, I am indeed a “Buffy-Watcher” aka B. Watcher, check it out if you want.
MST3K fans (Mystery Science Theater 3000, for anyone who may not have encountered this little gem of a show) are known as “MSTies” (pronounced “Misties”).
A videotape compiled of musical segments from various episodes was called “Play MSTie for Me”. 
I saw that they actually had a gathering for fans of “Trauma: Life in the ER” (a grisly but interesting TLC show). Hey, I find the show fascinating, but it never occurred to me to go to some CONVENTION for it. I can’t remember if Trauma fans had a special name or not.
Dr Who : Whovians