App103 is busy improving his already impressive to-do list program. This looks great, but for people like me who work on four or five different machines throughout the week (at work, at home, laptop, school, girlfriend's house), the installed software approach doesn't work.
Fortunately, there's a good free online alternative in Backpack. You can use Backpack for any number of things, but I use it for simple todo lists. It functions perfectly. It's one of my home tabs on Firefox, right between Gmail and my Markaboo bookmarks, at home and work. The folks who designed this also offer a whole bunch of other great software for people into GTD.
Other great web-based software stuff: http://www.37signals.com/