To-do lists are pretty important for getting things done; and much like text editors, every person has their own favorite flavor, and it seems like every developer has a slightly different idea of what an ideal to-do manager would look like.
Todo.ly is one take on the do-do list manager. It also comes as a Chrome Web app, so if you use multiple synchronized Chrome browsers (or Chrome OS) and install the Web app, you instantly get the same to-do list manager across all of your machines. Just that may be reason enough to set it up.
In terms of functionality, Todo.ly is pretty much what you'd expect. It supports due-dates and filters (Inbox, Today, Next). It also lets you divide your work into projects, which may have sub-projects. A task can also have sub-tasks.
One area where Todo.ly is lacking is linguistic processing: If I enter "Do something tomorrow", it doesn't set the due date accordingly. It also doesn't support hash tags, which is too bad (at least for me - I love hash tags).
Todo.ly is not team-oriented at the moment: You can't assign tasks to other people, or receive tasks. I guess they're saving that one for a future paid version.
I think it's pretty safe to say that most people who are not using some kind of stand-alone todo list are using their email inboxes as their de facto todo list. While that clearly works for some people, it's not kosher in the Getting Things Done universe.
If you're a Gmail user and you're finding yourself overwhelmed with trying to keep track of things in your inbox, but don't really want yet another place to check on things, consider trying GTDInbox. GTDInbox is a Firefox Add-on that adds todo list functionality to Gmail that is far more functional than the pathetic Tasks functionality that is built in to Gmail.
The functionality of GTDInbox is impressive. It uses a set of pre-defined labels to track the tasks that are sourced from email messages. You can create new tasks that are not linked to email messages, and track them all in a very Getting Things Done compatible environment.