My grocery hauler is a Townie 21, front-hub motor, and panniers. I can fit 3+ bags (cloth bags, natch) of groceries if I take along a back pack. This is pretty good but still I find myself a little short of space. (If I get an extra quart of milk will everything still fit?) Also if I put more than 30lbs or so into the panniers the handling gets more than a little squirrelly. This spring I went looking for a trailer to up the cargo capacity and maximum weight load. After discarding several choices and too expensive (Burly) or too flimsy looking (many others) I settled on the Nashbar Cargo Trailer from Bike Nashbar. It retails for $150 but is on sale at least half the time for $100 plus shipping.
The trailer mount replaces the rear skewer so it is incompatible with rear-hub motors. This is not a bad thing for me because the very last thing I need on this bike is more weight on the back wheel. Once the mount is in place the trailer pops on and off in a few seconds. Sweet!
I've used the trailer a few times now I can strongly recommend it. The advertised weight limit is 45lbs and a couple of boxes of cat litter have put me pretty darn close to that. About the only thing I notice is getting going is a bit harder with a full load. Glad I have a motor. :-) I have pretty decent brakes so stopping hasn't been an issue yet. Best of all it has eliminated my second to last use of my wife's stinking gasser - epic grocery runs. The only thing I haven't figured out how to haul with the bike yet is lumber... but I'm working on it.