So if you want to just read the code and make an educated guess, and throw money at it, the orange ones at Auto Zone and Advance Auto Parts will do. But if you want to do diagnostics and look at sensor values then you run into issues. Each manufacture has great scan tools for their brand. Snap On makes one that does alot but not everything. NAPA has a good one also but it has its limitations. It depends on what years and what brand. Sorry but its a very stressful occupation! Birdtracker
What are you going to use it for? What vehicles and how involved are you going to get. The cost is a big factor when choosing such a tool. If you just want to pull codes and look at some generic PID data, you can get a tool at Harbour Freight for about $100 on sale. If you are going to do manufacturer specific diagnostics, you would be looking at the tool that the individual manufacturer uses. You would have access to a LOT more information AND functions with a "brand specific" (Factory) tool. There are chassis, body, and other modules that would not usually be accessed by a generic OBDII tool.
I purchased a scangauge for my 2003 Trailblazer and it is well worth the money. It will give you codes and provide a monitoring system for your vehicle. It provides a code reader and reset, and gauges for watching various engine/transmission data. I use it to watch the transmission temp, water temp, MPG, Volts, and other info.
I have used it in other vehicles to read and reset error codes.