Kelty's Noahs Tarps are relatively inexpensive, pretty lightweight for not being silnylon, cuben, etc. and are well made. Only come in white, though. They're also square in shape which allows you a lot of versatility.
If you are willing to spend more money you can get some great silnylon and silcordura tarps from companies like Mountain Laurel Designs, GoLite, Etowah Outfitters, Sea to Summit, Granite Gear, etc. These are tough materials that weigh little but are impressively tough. But then you need to decide if you want square or rectangular, catenary cut, etc. Regardless, avoid grommet attachment points as they tend to tear out.