My Gmail inbox and calendar are where I organise my life. I find that around half of the things I have to do come to me via email, so for the other half, I simply send an email to myself so those things are also captured in my inbox. I use Google Calendar for to dos that are time based, eg. I have to do something at a particular time or on a date in 2 weeks time.
As you might just be able to make out in the screenshot below, I use the 'Superstars' gmail labs features. Red stars are important/urgent to do's, blue stars are less important/not urgent to do's and yellow exclamation marks are things I'm waiting to hear back from someone else about - this reminds me to follow up if people haven't gotten back to me about something important. Anything that can be done in under 3 minutes I do immediately rather than emailing myself or starring it.
I also use the 'Multiple Inboxes' labs feature so that my starred items, or my to do list appear on the right side of my inbox for easy reference.
I don't archive messages which I know is a little abnormal. If something is read and not starred it's the equivalent of being archived in my mind, but I like to keep it in my inbox in case I need to quickly scan and find an email that I hadn't needed to star earlier (though I love and often use Gmail search for this).
This approach came about after reading The 7 Habits of Highly Effective People a few years ago, then doing a Getting Things Done course at Google and speaking to another Googler, Will Blott who uses a system very similar to the one I now use.
While this approach works very well for me, everyone's different and I love hearing about other people's to do systems as I often pinch little things to make my own process more efficient. How do you manage your to dos?