Power Meters / Powermeters (SRM, Powertap, Quarq, Ergomo, Vector, Stages, power2max, P2M, 4iii, InPower, Cinch)

Measures oxygen content of the blood/oxygen uptake and uses that as an analogue for your lactate threshold. Do a ramp test whilst wearing it and it tells you when you tip over into anaerobic effort, in three minutes rather than 20 or 60.