For more than 3 years, I have trusted Toodledo to keep track of all my jobs and errands for me. Using one of the tenets of GTD you need to find a “trusted system” and this is it for me. I can enter, view and update tasks at their website, or in various widgets, or in their iOS app, or by SMS, twitter, email or even command line. You can get the tasks to show up in Calendars, have priority ones emailed or texted to you, and there’s also a nifty booklet printing option. The app tracks them for me so I don’t have to fill my head trying and failing to remember all of them.
The web version is free, but power users can pay $14.95/year for more features still, including the ability to collaborate and share tasks to and from others in your team or family. The companion iOS app is $2.99, and does almost everything the website does, whilst being offline. Both it and the website have been rock solid for me, and I have never had a problem syncing between the two.
For some reason it has never made as much splash as Remember The Milk or Things, but to me it has more consistently focused on adding useful new features – recently the ability to track real-world locations where you need to do jobs (for example a particular specialist shop, or your parents’ house). When you add in the iPhone app, it will give you proximity alerts to remind you that there are things you need to do there.
I have looked at most of the to-do lists apps that have appeared over the years, and tried a few out for a while. I haven’t yet regretted my decision to stick with Toodledo.