I have a bunch of iPad drawing apps (which is a little odd because I don't draw). The one I like for doing quick diagrams is Jot! Whiteboard by Tabula Rasa. There are both paid and free versions. (I also use Google Docs drawing program for more "formal" diagrams.)
What I like best is that you can move/copy/delete things (or groups of things) after you've drawn them. That's hard or impossible with a lot of "paint" type programs.
You can also easily add text boxes, which is handy for the kind of diagrams I draw.
Here's a couple of examples (the meaning isn't important)