On the home page there is a good article about different ways to rig a tarp.
http://www.equipped.org/tarp-shelters.htm
Tarps are pretty versatile. The polytarps are cheap enough but they are pretty bulky.
Mountain Equipment Co-op sells good light ones in silicone treated nylon.
A bit expensive for throwing over boats, cars, or patching roofs and windows, but great for backpackers to rig as a tent substitute.
You can likely find them for less money if you shop around. Just be sure to buy one that is big enough to actually shelter you, your people and your gear.
I like this one:
http://www.mec.ca/Products/product_detail.jsp?PRODUCT%3C%3Eprd_id=845524442505763&FOLDER%3C%3Efolder_id=2534374302702975&bmUID=1231466409108

10x12 seems to work for me.
But to each his own.
If you go with a tarp you need plenty of good cord with it.

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May set off to explore without any sense of direction or how to return.