Hey Joe,
great topic!
There are several that I like. My favorite is one used to train for mulitple attackers (this seems to come up a lot). Though there may be slight differences in the techniques being trained/employed, basically you train with either 2 or 3 uke(s). Each attacks from a different angle, with a different aggressive act (round house punch, snap kick, and from the rear maybe a bear hug, rear naked choke, or full nelson for example.) I personally prefer that one is straight on, one comes from etiher side and one from the rear. The attack from the "rear" may involve any number of defensive tactics. Though I mentioned chokes, etc., it could just as easily mean doing a back kick, spinning backhand, or just moving and getting squared off to deal with him/her.
How quickly the attacks come from the various ukes varies with the skill of the tori. (It should be increasing more difficult with moer skilled practitioners.) Everyone gets a try at uke and tori as you run through the drill, the same attacks and defenses are used by everyone. It is not a "free" sparring session. Once bodies start moving and going to the floor, etc., people will quickly see how important is to percieve what is going on around them and adjust accordingly.
The exercise helps in a lot of abvious ways in terms of techniques but it also helps build a "360* awareness". It doesn't take much imagination to see how to make it an interesting part of a class.