if you have a VW/Audi, a "VAG-COM" is the superior product - shows you lots of factory codes/data. Support forums....excellent tool.
ross-tech is a 3rd party company (ie- they hacked into VW's ecu programs) but its a mature product and I've never heard of any safety/reliability issues - it just lets you read all the factory codes (that are more far reaching than teh generic OBDII codes.
http://www.ross-tech.com/ <- they make VAG-COM's
If its not VW or audi - I'm not sure. You'd think that every manuf. has a proprietary tool....
-Johnh