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:

1. List (eg. Personal, Work)
2. Location (eg. 1455 Grand Ave)
3. 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.