I've been trying to use RememberTheMilk as my one trusted system, even though I am very undisciplined personally. I think one of the reasons RTM is working for me is because there are so many ways to interact with it:
- web
- offline with google gears
- mobile web
- milksync
- email (can enter tasks with text messages this way)
- Gmail greasemonkey plugin to manage tasks from inside gmail
- iGoogle plugin
- and now, Tasky! Tasky even integrates with tomboy
- with the recent launch of google gears for mobile, I'm sure the mobile web interface will get offline support soon