Fanfiction
I started writing fanfiction when I was in college, about ten years ago. A friend got me reading Harry Potter fanfiction in high school, but I only ever read what she recommended, which was well written. It wasn't until much later that I realized how many poorly-written fanfictions there are out there, and I got tired of trying to sift through it all. So, I don't read it often, but I can get in the mood for it.
I didn't post my first fanfiction until about a year ago, and I've only posted four or five since (mostly in the StarGate fandom on fanfiction.net). I have a tendency to come up with plot ideas while I'm watching a series, and I start writing it, but as the show plays out, my idea isn't possible with the canon, or I lose interest after I'm done watching the show. So, I end up with a bunch of half-finished fanfics on my computer.
Even without finishing them, I enjoy writing fanfiction, as it gives you parameters - already established characters and worlds - and you get to play within them. It makes for good writing exercises without the hassle of coming up with your own original characters and situations. I like to use it to practice certain parts of writing - dialogue, setting the scene, etc. It takes a lot of the pressure off if you don't feel like you have to introduce everything like you do when it's original. It takes a lot of the "work" out of writing and allows you to just have fun and write.