Check out the Otterbox Commuter or Diztronics TPU cases. Both are excellent choices IMO.
http://tpu-cases.com/index.php?main_page=index&cPath=42_68&zenid=1c01943b21170ac1a4bf231ce9c22cd6
http://www.otterbox.com/Samsung-Galaxy-S3-Commuter-Series/SAM4-GALAXY-S3,default,pd.html?dwvar_SAM4-GALAXY-S3_color=20&start=3&cgid=samsung-galaxy-s3-cases