Earlier Monday one of my colleagues from Gamespot spent most of lunch gushing to me about his new favorite GTD tool. Called Toodledo, it's diminutive name does not do its to-do list prowess justice--this is one of the most deep and full-featured offerings on the market. It's also one of the easiest to get into, especially if you're using other Web services like Google Calendar, Twitter, and Jott.
At its heart Toodledo is a task organizer, so two of the most important aspects should be entering in the data as well as being able to access it from all over the place. Luckily it does a great job on both counts. You can plug into your task list from all sorts of places including mobile phones, start pages like Netvibes and MyYahoo, Twitter, and on various widget engines like OS X's Dashboard and Vista's Sidebar. In any case the interface is pretty familiar: just a simple rundown of what you have to do and some empty boxes to check off whatever you've dealt with.
Two of Toodledo's most handy input methods are actually outside of its core Web service. Using speech-to-text service Jott you can simply call in and leave a to-do item. It will convert your call into one or more to-do items while managing to pull out any dates and times. Having used this with ReQall's iPhone application (coverage) it's just plain handy, albeit a minute killer if you're on a tight cellular plan.
The other method I like is the Firefox extension that lets you create and manage list items without having to use a separate application, or you keeping the site open in another tab. It also includes a contextual menu shortcut, so say you get an e-mail from a friend about their favorite wine, you can simply highlight the name, and right click to send it to your to-buy list. You can also do this with entire chunks of text and it will simply pull the dates and add the entire clipboard into the notes section of that item.
Also of note is the iPhone Web app, which made waves for being one of the first to-do lists to get optimization for Safari mobile. What makes it cool is that you can enter items even while you're away from a data connection, as long as you've got the entire page loaded. This isn't as good of a solution as a native application--something that could give you reminders, notifications, and be accessible offline, but it's still quite handy as its own management system.
Toodledo has far more features than you're bound to use. Those looking for more, including a file storage system for group to-do collaboration, as well as an analytics system that crunches through your task history to find trends, can be had with two premium plans that run $15 and $30 a year respectively. You can see a full breakdown of what's included and what's not, along with what the competitors have to offer on this page.