I'm thinking about this very simple device for my next car project: a 4-led lambda meter that provides simple and readable information on your air/fuel ratio. Nice for turbocharged cars, and only $60.
Compared to the $1000 MoTeC unit, it's a little more affordable, although it's not a true 5-wire NGK/NTK UEGO controller. An even more simple approach of literally using a voltmeter may be another alternative. Or there's always Autometer. Also: