From the GTD book: "To manage actionable things, you will need a list of projects, storage or files for project plans and materials, a calendar, a list of reminders of next actions, and a list of reminders of things you're waiting for..."

Of course, the beauty of the system is that you can tweak it any old which way, and of coruse the tickler is a great place to put things you're waiting for that you don't need to see until a particular day.

I find the list allows me to check in and see if I've been waiting for too long, and then I can nudge as appropriate.