I carry one of those plastic tarps with gromets. Think it's about 5'x7'. I have only used it once as an additional shelter so I could cook and sit outside the tent during all day ntermitent rain and drizzle. I simply hung it up between trees using simply twine I had brought with me. I don't bring the tarp solely for an additional shelter. I find it quite useful for other things, like keeping your butt clean and dry. Easter weekend we did an overnight trip to a place that has gritty dirt camp sites. It had rained hard earlier in the week and the ground below the surface was saturated. Had we not had the tarp to sit on we would not have been able to sit by the campfire ring without getting wet and dirty. I have also used it to protect gear left out overnight on picnic tables from the rain.

When not in use, I fold it up into a rectangle and it sits nicely under my tent on my rear rack:

http://www.flickr.com/photos/davez2007/4491664846/

Some will likely say that it's useless weight. But I'll carry the 1 lb. tarp, shed 1 lb. of body fat and enjoy the convenience.