There is a lot of talk here about the benefits of going tentless and adoping a tarp, especially the dual use poncho/tarp which has a hood but can be pitched like a tent. I have been researching this subject and found that light tarps are surprisingly expensive, especially catenary cut ones.
Ive been holding off and holding off, until today I realized I HAVE ONE already. My old army poncho is PVC coated ripstop nylon, with hood, grommets on corners and mid-way on two edges.
These can be had for $25 or less. Search for 'military rain poncho' on froogle. 'Fakes' weigh less, but the real ones last longer. The weight/durability tradeoff again.