Always use renedra plastic bases, except for bigger bases where I use plasticard, just looks neater than wood to my eye. I always buy direct from renedra, very efficeient and friendly service, and they take PayPal.
Rufus T. Firefly: Oh, I'm sick of messages from the front. Don't we ever get a message from the side?
I use laser cut MDF from East Riding Miniatures. No trouble with warping so far and I've used hundreds basing my Napoleonics and ACW, with water based filler to build up the ground. Delivery is very fast too. I have had trouble with plywood warping in the past so I wouldn't recommend that.
I just found this place that does some nice bases with rounded corners.