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.
blackbootz The key insight that I think kb refers to and that I got out of GTD is that it is essential to develop some kind of system first and foremost. Whatever site / app / book you use should fit that system best. For example, my approach to email is that I make a Todoist to-do out of everything that asks me to do something. I archive all the other emails and only archive emails with actionable items once I did that task. Simple system + Gmail + Todoist = clean inbox. Might be a good idea to reflect on where your obligations come from and what systematic way you can tackle them - e.g., when you get a text with something to do, where does that thing go?
You know I hate books :p So I actually own a copy. The top review in Amazon nailed the issue on the head for me: The book is 80% filler. Page after page of rehashing detracted from my motivation to finish it when I picked it up last year. But I'm a lot more motivated to glean its lessons now that I'm roughly 10 times busier. I'll crack it open.