I'm looking for a free, in-depth task manager/to-do/list application. Does anybody know of anything?

I'm looking for something to manage all the things I have to do, but not too basic. Something like Stickies but with a to-do function, possibly?

Edit: I just wanted to clarify that I'm looking for a free application (preferably not web-based) for my MacBook. I appreciate all your suggestions and I'm trying them out as they come. Keep them coming and I'll let you guys know which one is the best or if I find what I'm looking for. Thanks!