Unleashed
The site that inspired this one. Brilliant MSTs! Not just funny, but also incredibly biting.
Mystery Hogwarts Dungeon 3K
Abysmal fanfic, this site has been around for awhile and they keep getting funnier EVERY FUCKING TIME I READ 'EM!
Harry Potter MiSTs yahoo group
Just what it says. Though it seems that this group only accepts MSTs hosted by the MPP. No, not MWPP because Wormtail is far too great to be included with such a band of dismal failures.
MSTs are Fun (and other stories)
A site dedicated to getting all the HP MSTs from ff.net within it. Unfortunately, they don't seem to have any hosted by the Slyths but we'll give 'em credit for trying.
Roleplaying Horrors
These guys have taken the LJ community bad_rpers_suck to the next level. Not HP MSTs but just as good. Hell, they actually like this site. You've got to give them credit for that.
Dark Shaftings
The one Lucius archive!
Enticing Lies
Lucius' player's massive fanfic archive! If you don't go there, then your fic will be MSTed due to the laws of karma.
Mirror Cracked
Severus' player's fanfic, fanart, etc archive! Go there for the only IC Snape EVER!