Looking back, I'm not sure anyone mentioned their favorite tarps.

My favorite is the Cooke Custom Sewing Tundra Tarp, made the U.S. by a great little company in God's country (northern Minnesota). Mine is a 10'x12' 1.5 oz. $110 is a great price considering the quality.

Here is their main link:
http://www.cookecustomsewing.com

Here is the CCS Tundra Tarp:
http://www.cookecustomsewing.com/tundratarp.htm

Some nice pictures here:
http://www.bwca.com/index.cfm?fuseaction=forum.thread&threadId=94319&forumID=15&confID=1

While you're there, check out their Lean shelters. I don't own one yet, but I REALLY want one.
http://www.cookecustomsewing.com/leans.htm