I carry a $16 'GI Style' ripstop poncho that I picked up at a local camping/surplus store. It is a little smaller (I think) than an actual GI poncho at 56" x 90", but it is durable and has covered my hammock on numerous occasions.
Lately, I have been considering an extended_length silicone-impregnated nylon poncho. This would offer me a little more coverage, and weigh as much, if not maybe less, than my current poncho.