This is the fastest and easiest pitch I know. It requires a tarp, a tree, and three stakes (optional) A flat 8'x10' tarp is pictured here, though a 6'x8' would also work.
First find a tree with a nice spot to one side of it. The space needs to extend a fair distance from the tree, about ten feet.
Tie one corner of your tarp to the tree at about head height. Then, grab the diagonally opposed corner and stake it out as far away from the tree as you can. If you don't have any stakes just hold it down with a log or rock, or even tie it to some bushes. This is a low tension pitch so it's fairly forgiving. Then take the two remaining corners and stake or tie them to the ground so everything is taut.
Voila! You have a shelter that will comfortably sleep two. An yes, that's a very car shaped tree... it's just the picture I had laying around.
Note: Your head will be at the tree side of the shelter, so make sure that side is up hill unless you like inversions.
Unfortunately side blown rain will still get you wet in this shelter, and a heavy wind from the tree side of the shelter would put a lot of strain on the pitch and possibly collapse it. I've never had that happen, but I don't use this pitch in storms.
This shelter works best in the same weather where an umbrella would keep you dry. That is to say it's useful for rain in still air, and for keeping off heavy dews. I learned this pitch in Seattle, where the weather is almost always perfect for it. It's also great for dealing with fog coming off the ocean, which is key down in California. In general the openness makes it ideal to cook under and it's tall enough to sit up in, but my favorite thing about this pitch is that almost half the setup time is pulling the supplies out of my pack.