It's kinda your best friend.

Many of the sites linked to from this one are great for putting up or reading drabbles. Unfortunately, none can match the options and variety offered on Livejournal. The website not only offers everyday discussion groups and a way for you to constantly bombard friends with your thoughts, but a limitless realm of opportunity for drabble writers and the higher intellect most fandom writers crave for like a freezing cold ice cream sundae in the middle of July in Arizona (not that other hot and arid states don't want ice cream sundaes, I personally think Arizona sounds nicer...and it's the first thing that came to mind).

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Now that I've promoted my butt off:

go join! But seriously, if you don't already have an account I really recommend getting one. (go here) A basic trial is free, though you could end up buying one later if you enjoy the site and so choose.

If you already have an account/after you join:

Find many great drabble communities by doing an interest search for '100' or simply 'drabble' or if you're really adventurous try 'drabbles'. Either way, you'll find hundreds of communities and somewhere there has to be one for you. Even normal fandom communities have at least one drabble challenge a month or so. The opportunities are there. Not only that, but they're prevalent. SO GO ENJOY!!

(Oh, it's not being shamelessly promoted at all. *whistles*)