Okay, I'll admit it. I'm addicted... to Backpack.
This is just about the smoothest web application I've ever seen. It's AJAX'd to the hilt (which means no page refreshes for all you non-techies). It has a great balance of features for creating lists, taking notes and packing your files around to different machines. I used to e-mail reminders between my work and home machines. No longer. Backpack's my homepage both places, so I see what I need to remember. You can share pages too, so my wife and I can pass lists of, for example, groceries, back and forth during the workday.
Backpack has also carved its way into my writing process, but I'll scribble down more about that another time.
I really admire their design philosophy -- they don't put in features or settings simply because some people ask. They really consider what they're doing, and are committed to doing it one way and doing it right.
That isn't to say they don't add features, though. In the time I've been using the service, they've made substantial strides in the ordering for lists and notes, and then just today bumped up the size and page counts for paying accounts. Hurray!
Anyway, I can't say enough good things about it. Go check it out.