Well, my first thought was to mail them around. (My local Blockbuster is starting to use a different type of storage sleeve, so I've been able to snag some of the old plastic ones, which look ideal for mailing. You can also purchase them.)

Alternatively, it was a matter of passing them on from person to person, with the expectation that they would eventually find their way home.

Finally, it occurs to me that SE's could probably put the concierge service to work, although I'm not certain how they would feel being used as "librarians."

I wasn't thinking of direct swaps/trades or loans, but rather an informal network to share DVDs. The only time I'd likely use them is on airplanes.