There's a lot of filk out there in the world; I've written some of my own. It all started for me after I saw the big success X-Men had become ... AFTER I'd sold my collection of 200 consecutive issues (I was tired of some of the plot convolutions).
Later, I wrote a Babylon 5 filk (concerning G'Kar).
The explosion, however, came when I started playing a bard in Dungeons and Dragons: Roscoe's Songs
I wrote a filk about configuration management (Safety Branch), but unfortunately the forum I posted it on died; I haven't recreated it yet. You can branch if you want to, you can leave that trunk behind...