Remember The Milk (http://www.rememberthemilk.com) has some very powerful features including Locations, tags, Smart Lists, offline support, and advanced searching. I often share tasks with friends, but sharing does have limitations. The following fields are not updated to the person you share with when you make a change:
- List (eg. Personal, Work)
- Location (eg. 1455 Grand Ave)
- Priority (eg 1,2,3)
However, other fields such as tags, due date, title, and Notes are updated in real time to the other person when you make a change.