Places to post Fic

 Fanfiction.Net -also known as the pit of voles. But by its ease of use and sheer volume alone, it wins everytime. 
Adultfanfiction.Net - obviously specializes in NC-17 rated fic. (Yay, porn!)
FictionPress.Com - original fiction only (no fan fiction)
Skyehawke.com - has only a few categories, but X-Men and Harry Potter are among them. 
FanWorks.org - smaller archive, but tons of categories
FanDomination.net - deals in real people fic or actor fic. which is quite naughty, and thus very fun. (not a lot there though)
FictionAlley.Org - Founded after the infamous Cassandra Claire incident. Got kicked off of FF.net? Post here. (Harry Potter only)
The Wonderful World of MakeBelieve - an older, retro-looking site. But seems to be thriving with lots of fic
The Definitive X-Men Erotica Archive - mostly comic book characters, all NC-17
The X-Verse - a whole community of forums that accepts fanfic
Livejournal.com - duh! post to your journal, communities, etc. (more on xmen specific stuff below)

Fic & Related Communities

_fanfics_ - FanFiction Archives
fanfictionet - Post fanfic from anywhere or advertise your community
allfandomfics - All fandoms, all fics
fan_fiction - All fandom fiction site
fics - Mostly anime fics, but offers critique
crossficathon - Runs a crossover Ficathon
dear_multiverse - Answers questions about all sorts of multiverses for a bunch of comics
drabble_madness - Accepts drabbles from basically everything ever written/put on tv or made in a movie
fanficswapmeet - Fanfic Swap Meet
fanfic_fiesta - Come and be inspired
femslash_fluff - F/F pairing Fluff!
fics_on_request - Request a fic, or write a request
promptsomeslash - allows you to ask and request slash pairings
ship_manifesto - Essays on pairings in all Fandoms
fanthropology - The meta study of Fandom
crack_van - Recs good fic in several Fandoms

X-Men fic Communities

marvel_aoa - Age of Apocalypse
marvel_fic - Marvel Comics Fanfic
marvelously_ooc - Marvel-ously Out of Character
xmmff - X-Men Movie Fan Fiction
x3fic - Fanfic for the Last Stand
xmenfic - community affliated with WWOMB

Characters or Pairings

magnetic_appeal - Magneto community (mostly comics)
oldfriends - Xavier/Magneto slash fans
xavierfans - Charlie's Angels'
jadore_lebeau - Remy LeBeau community
bobbykitty - Yes, even Bobby and Kitty have a community.
icevsfire - Iceman/Pyro Community
fast_love - Fast Love ~ a Pietro Maximoff community
kittyandpyro - just a boy and girl and a broken love story
loganandmarie - Wolerine and Rogue
ragin_remy- Another Gambit Remy LeBeau Community
regan_omics - Remy and Logan (did not see that one coming)
whitequeencourt - Emma Frost
demented_allure - Rogue and Magneto love
cruellyhandsome - A Magneto/Mystique Community (run by me!)
bh_slash - Brotherhood Slash
raven_darkholme - Mystique community, largely abandoned

X-Men Non-Fic Communities

x_movie_stamp - Find out with X-Movie character you are most like

Fan Fiction Resources

 How to Write Better Fan Fiction - The Best resource around. Organized into hundreds of tips about writing and fan fiction in general. very useful.
The Fanfic Symposium - Useful articles on how-to, op-ed pieces, and history of fanfiction
Pacing In Dialogue and Action Scenes
Why Chicks Love Fanfic Porn
Snowflake Method for Writing a Novel - not fanfic related exactly, but the basic idea may be useful
Fanfiction Glossary - Confused by terminology? Unsure if you're writing a Mary Sue? What does fen mean? Go here
A Gallery of Archetypes - creating a character? Take a look at the classic character types first. 


These lists are by no means comprehensive. If you know of any more places/helpful stuff. Reply and I'll add them on. If you like these lists, tell your friends, add it to your memories.