I use a WBO2 from http://www.14point7.com when I tune cars for other people.

I use the SLC_DIY, since it comes in kit form, and I like to work with electronics, and it has a lot of very cool features. It is only one of two WBO2 controllers that I can find that has native USB, which is important with laptops not being made with serial ports any more, and it has a built in AFR display. There are also options for thermal couple (EGT), manifold pressure, and 7 addition 0 to 5 volt inputs. The price is extremely competitive as well, costing less than my other big name WBO2 sensor, with far more features.

You can also buy it in pre-assembled form for not much more.

Here is the complete kit:

Part way through assembly:

Assembled, before testing: