Got myself one of these for my 01 V70 T5 recently

http://www.auterraweb.com/

I got the Windows version supporting the newer CAN interface (Figure I might need it for future cars). Got mine on ebay for $210 (from Auterra themselves) incl/shipping. My OS V70 T5 uses ISO interface so I could have purchased the cheaper one (no CAN interface)

http://www.obdii.com/connector.html

The upside is you have the full use of a computer. I can record sensor readings, store and replay them. Big screen providing lots of info. Downside is you need your computer when you do this. To debug check engine lights that goes off after a restart means you need to drag your notebook with you in the car.

Since purchasing this tool, I've learned much more about what all these sensors do. I realize while there are definitely more engineering in the newer cars, the extra engineering is how we are getting more performance over a wider operating range. But it does make it more complex with additional failure points. Mechanical systems providers most mechanically inclined person a similar level of familiarity while electronics + software makes everything more invisible.

I also found this site very educational. While it is for Toyotas, there seems to be lots of similar designs in these modern cars.

http://www.autoshop101.com/
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2001 Volvo V70 T5 ASR Wife says we are Volvo for Life, 1992 BMW 535iA becoming The Ultimate Driving Machine