15minuteficlets - A site with a weekly challenge. One word, you look at the word and write for 15 minutes immediately thereafter.

400words - Terry Pratchett, when asked for writerly advice, said that he's written 400 words a day, every day, for the last 6 years. If he had nothing to say, he'd invent characters and have them talk to each other. Goal = 400 words a day.

anime100 - a weekly prompt which people write 100 words of fanfiction based on an anime series

bodyandsoul100 - 100 prompts, sign up and write 100 pieces (or draw 100 pieces) about an orignal character according to 100 themes specified in the userinfo.

book100 - weekly prompt which people write 100 words of fanfiction based on book or book series.

daily15 - a daily version of 15minuteficlets. One word, write for 15 minutes on it.

haiku_for_you - a community to write haiku and share them with the world. Fun and simple. Talent optional.

hp100 - weekly prompt which people write 100 words of fanfiction based on the Harry Potter series.

hundredwords - a challenge to write 100 words every day for 100 days straight. Miss a day? Start over.

shoujoai100 - like bodyandsoul100, 100 prompts to write a piece of fanfiction on. You must claim a character or pairiing from an anime and it must be a shoujo ai series or feature a shoujo ai pairing.

nh_nanowrimo - a New Hampshire Livejournal Nanowrimo group. Organized mostly for the off-season (not November).

rarelitslash - the place to post Slash written about obscure literature pairings and books.

snape100 - weekly prompt which people write 100 words of fanfiction about Snape from the Harry Potter series.

storyseeds - a writing prompt community. Meant to 'seed' your mind to write a story.

the_spirit_lamp - ocassional writing prompts geared towards self reflection and introspection.

tv100 - weekly prompt which people write 100 words of fanfiction about their favorite television shows.

utenadrabble - a drabble community based around the Shoujo Kakumei Utena series.

nanowrimo - the LJ community for National Novel Writing Month.

littledetails - a research community for writers to ask questions about subject areas they wish to learn more about.

out_lines - questionaires to be filled out as you develop characters.