You cannot beat the OBD II software and accessories from EASE diagnostics, for the money it is the best buy, 'cause once you get familiar with its multi functions and ability to manipulate and understand the data it returns, one can make all kinds of friends by scanning their cars, and an OBD II system such as this is a must for anyone living in areas that have emissions testing or WILL SOON have emissions testing. Also, the software will read all protocols and has tech support for upgrades, (which hand held models will not have,and are usually only for a narrow range of vehicles) Cool real time color graphics and charts, one can plug it in the dlc and drive around watching what the engine components are doing and how it responds to acceleration or decell, etc, All for around $500.
See their web site

For OBD I cars, somebody describe on this website how to make a diode light for like $15 you might try a search.