I've been using so-called GTD software and services to make my Calendar, ToDo and memo things online and most of them failed.
Back in 2000, I started writing up my todo in ~/todo.txt and setup a cronjob to email to my cellphone every day 10 AM. It actually worked really well, but i just wanted more calendarish feature, categorizing by tags and more priorities thing.
So far I tested, Basecamp (overkill for personal tasks), Backpack (Good for Wiki-like thing, not for ToDo), 30 boxes (good for calendar, but ToDo is poor), Remember the Milk (Good but the hot-key is complicated to remember) and Gmail (Not designed to be used as GTD software). But they all sucked and recently I manage them as a physical stickies on the wall of my desk, which works really well, but not searchable and reachable from my home, since it's not online.
So, recently I started using Hiveminder, the newest task management system from Best Practical Solutions, the creator of World #1 ticket system RT. To be honest I don't think the RT User interface is best in the world, but soon after I started using Hiveminder I felt "this is THE one I wanted."
Its UI is all Ajaxy and pretty easy to use. Doesn't work 100% well on IE, but works 90% and that's fine. "But first ..." and "And then ..." (tasks dependency mgmt) looks really useful, even though I never tested them yet. So far, it has everything I need for a personal task mgmt system and does syndication with Atom, plaintext and iCal. Sweet.
Now I'm using Hiveminder as Task mgmt, 30 boxes for Schedule and Gmail for personal memo. I think this is the best combo so far.
Hiveminder is now open for everyone. Give it a shot.