I definitely agree - challenges are my buddies. I mean, I spend a shitload of time whining to myself about them, and staring at a blank Word doc, hoping to even write an outline - but while I do come up with some things on my own, challenges get a lot of it out, too. Or because it's someone's birthday, or someone requested something - whatever. Most of my Dollhouse fic was for dollhousefics challenges, and I recently did a bunch of challenges, like plot_wout_porn.

But I do agree with other people - depending on what kind of brain you have, or what motivates you, some people can write things that stretch their limits and are difficult and challenging with just their own brains to inspire them.

And speaking of challenges, I actually wrote my joss_las already! Just need to revise it and then it's go. :O