I typically use Google Docs for this sort of thing, but I'm pretty old school. I just open a doc, and start writing down bullet points of features / behaviors that the application should have.
Once I've got all the high-level requirements in place, I then just start going through the list knocking stuff out, one by one.
Once I've got all the high-level requirements in place, I then just start going through the list knocking stuff out, one by one.