Web/iOS/Android/Mac: There's no shortage of task managers to choose from, including many that sync across multiple platforms. Doit.im stands out as being one of the easiest to use, while offering just about every feature a GTD-lover would want.
When we first took a look at Doit.im years ago, it was a desktop (Adobe AIR) download for Windows, Mac and Linux. Since then, the app has transformed into a web-based and mobile task manager. In addition to using Doit.im in the browser, you can access your to-do lists on the iPhone, iPad and Android, and there's a mini app for Mac OS X as well.
Doit.im, as before, supports the GTD method of task management, with next, someday and waiting categories, as well as built-in contexts such as "home" and "phone". Drag and drop your tasks to any bucket, add tags (and easily filter by them with one click in the menu bar) and group tasks into projects.
Even though it's a robust tool, Doit.im is really intuitive to use and easy on the eyes. It's similar to the popular Omnifocus app for Apple products, except free and cross-platform.
I spent a whole weekend looking for a new to-do app and Doit.im won its place as my app of choice -- for its cross-platform support, uncluttered interface, powerful filtering and tagging tools, and one feature most apps don't have: seeing your tasks on a monthly calendar that syncs with Google Calendar. (The also well-regarded Producteev has calendar viewing, but it doesn't seem to sync with Gcal anymore.) This means I can see my scheduled tasks along with my events and appointments at once in Google Calendar (or just the tasks in a monthly view in Doit.im) -- something I really need to make sure I don't overbook my days and so I can visually plan my schedule.
There is a downside, of course. While Doit.im is free, only certain features are available in a Pro account ($US2 per month or $US20 per year). These include continuous sync (the free version syncs across your devices only once every 24 hours), adding tasks via email, and task reviews/reports. The Pro version also adds priority support, beta trial privileges, a Pro version for Mac (in addition to the mini app), and quick-add features in the mobile apps.
If you haven't taken a look at the latest version of Doit.im lately, I think you'll be pleasantly surprised.