Rather than recommend an app or a thing, can I recommend a book?
GTD is a cult that has waned in popularity but still has zealous proponents that ascribe far too many powers to it but there's a kernel of truth to it that can be gleaned by reading the book in an afternoon. The trick isn't in finding the best way to track your obligations, the trick is in finding the best way to complete your obligations and what works for someone else may not work for you.
For me, the simple, kitschy stuff really does work - having a label maker makes me more organized. Having a file with 12 months in it gives me power over my receipts. Etc.