A text file called ideas.txt, one idea per line. I try to aim for just a stream of consciousness; if anyone actually read it I'd be quite embarrassed. The fewer formalities or barriers that exist between a momentary idea and writing it down, the better.

Ideally, I should have placed the file under version control from the start so I could reference when a particular idea came to mind, but I didn't do that.

My actual project planning docs are all text files though, and those actually are under version control. I find using a blank commit message works best because it lowers the barrier to further editing or writing.