[nickcharlton]

I personally use Things (http://culturedcode.com) with a simple "Projects" project created to dump them in. (at least, for directly recording)

However, I also use a whiteboard to scribble stuff on and these can be a great solution for presenting ideas, especially if you can add to them as you present.

I would suggest Balsamiq Mockups too, especially for drawing up interfaces, although you might find that using a "thinner" solution better for you. (Mostly related to the size of the elements in Balsamiq.

[dot]

I've been using Things to keep track of ideas as well.

Actually, I've started using things for all kinds of notes and code snippets. I love it.