When I collaborate with another person we sometimes agree that he or she will do a particular task. I am often responsible for seeing that the task gets done so I don't want to lose track of it but I don't want it in my "To Do" list.

I use a tag called "assigned" as well as a tag with the name of the person who agreed to do the task. I use Smart Lists for my To Do lists and then use not tag:assigned.

For example:
(priority:1 or priority:2) and not tag:assigned

Thus the assigned item can still be priority 1 but not in my To Do list.

More generally, all items tagged 'assigned' can be kept off of any smart list (such as shopping) like this:
(the rest of the smart list definition) and not tag:assigned

When I get together with a person to whom I have 'assigned' items, I can select the tag with his or her name to see the things we might want to discuss. This method is particularly handy on my mobile device.