Here's a good start:-
http://www.freelance.primetap.com/backyard.htm
...and some more basic rules:-
Backyard Cricket
Rules
The rules of backyard cricket will vary with every backyard, but here is a list that many a game is based on.
1.One hand off the house: Catches must be taken one-hand if the ball comes off the house/tree/fence.
2.Six and out: If you hit it over the fence you're out and you have to get the ball.
3.No LBWs: Too contentious, unless dad agrees to umpire.
4.You can't go out first ball: This is to stop your kid brother from blubbing or to give granny a chance.
5.Last man's tucker: You can bat by yourself if you are the last remaining batsman.
6.Tippity run: You have to run if you hit it.
7.Magic wickets: You can run out a batsman batting alone by hitting the wicket at either end.
8.Automatic runs: If you hit certain backyard landmarks - a fence or shed perhaps - you are awarded a prearranged number of runs