Remember the Milk (http://www.rememberthemilk.com) is a free, powerful on-line to-do task reminder application. You can create multiple lists of to-do items and assign due-dates using “natural language” (i.e. “Tomorrow” or “Next Tuesday”). You can also assign priorities, time estimates, locations and URL’s to each item. You can also tag to-do items, and then create “Smart Lists” that collect similarly-tagged items, regardless of which list they appear on.
For a small fee ($25 per year) you can upgrade to the Pro edition, which allows you to easily sync your on-line to-do list with your iPhone, BlackBerry, Android, or Windows Mobile phone. This integration is quick, easy, and is two-way (adding or updating items on your phone causes those same changes in the on-line version, and vice-versa).