Since common front or rear hub kits obviously will fit any bike, no additional info for them is needed here.
The currently best-known power system for cargobikes is from a company called Clever-Chimp and its called "Stoke-Monkey". I believe its a de-spoked Crystalyte 407, which directly drives an extra chainring on the Bottom-Bracket (BB, the pedal axle). The system has proven to be very popular, and the only complaint I've read is that the chainring does not freewheel when using E-power. As a result, your feet must always be pedaling when under power.
The Crystalyte 40X series is no longer retailed in North America, and it is well-known to run very quietly. Driving the rear wheel through the BB results in the motor having the use of the rear wheels gears. This helps hill climbing and motor/controller heat a great deal. As of Nov 2011, Crystalyte is selling the new G-series, which may be an option. It is a direct-drive hub similar in size to the old 40X, but it has not been verified to work as a non-hub mid-drive yet. https://endless-sphere.com/forums/viewto ... =3&t=34203
The urbancommuterstore.com is about to start selling a similar system (scheduled to be available by the end of Oct 2011). Theirs requires no changes to the BB or chainrings, and since it adds a freewheeling jackshaft, the pedals are not required to turn when under power.
https://endless-sphere.com/forums/viewto ... 60#p445575
https://endless-sphere.com/forums/viewto ... 28&t=35008
Driving the BB with a retail non-hub motor kit was pioneered by Cyclone. They are still a viable option, though some customers have been disappointed by its loud noise and also various components being weak, and usually requires upgrading replacement parts soon after initial use. If you are interested in a Cyclone kit, I highly recommend getting the 650W system or larger, as the controller is separate from the motor, and they tolerate heat much better. The separate controller/motor also makes repairs and upgrades easier.
Ecospeed is another company that makes a BB-drive