I loved following the story last week of the Mad Men Twitter accounts, where people blogging as "Don Draper" and "Peggy Olson" were shut down by AMC (which later relented and allowed the feeds to stand). What I hadn't really thought about — until the New York Times pointed it out — is that the Twitter feeds are a new form of fan fiction.
I'm more familiar with the older idea of fanfic (as the Times put it: "Not satisfied with the number of original “Star Trek” episodes? Why not write a sonnet about Captain Kirk’s feelings toward Tribbles and post it on your blog?"). It's a pretty interesting idea to take characters from your favorite shows, books, or movies and make your own stories with them, and some fan communities (Twilight fans, Buffy fans) are particularly prolific. I've read some fanfic in my day, but I can't say I've ever written any. What about you?