Folks one more little vote for a Google mail extension called Active Inbox.
It's not nearly as powerful as the systems referenced - Omnifocus, Asana, ToodleDo, Trello, etc etc. It doesn't have cool interfaces or charts.
BUT it directly interacts with email, which is still my #1 inbound 'what is it?' source. Basically it harnesses Google 'task' inside GMAIL with labeling and tiering to create lists/projects. It's designed by GTDers.
My email INBOX is still my GTD INBOX and I can't foresee a time when that will be different.
If anyone else has resolved the INBOX-INBOX dilemma with software that then will bring the more intuitive and powerful GTD supports, I'm all for that, and then integrating that with GlassFrog