I have outline writer's block. I can outline essays and blog posts, but never fiction. I think a big part of that is that the scenes that I write motivate and inspire the scenes I write in the future. I guess that makes me a discovery writer.
Before I heard the term "discovery writer", I thought I did pretty good outlines. Except, well, they were just all in my head. I build up a mental picture of the world, an understanding of the characters, and key events from the beginning, middle, and end; then, I just start writing. I'd say my conception of the plot is vaguest at the beginning; I need to have the world and characters nailed down pretty tight before I start writing, and the plot grows out of that (and, recursively, out of itself).
If I ever start writing novels, I'll need to write more of this stuff down because it's not all going to fit in my head, and it'll need to stay consistent (stuff held in wetware tends to evolve without my noticing). But as long as I'm still just doing shorties, I can keep refusing to use any sort of formal outline.