While GPS has revolutionized the way we navigate in two dimensions, the most common cause of pilot violations and CFIT accidents is the pilotís failure to deal properly with the third dimension: altitude. In this regard, a good altitude alerter can be worth its weight in gold. Once the exclusive province of the turbine crowd, alerters have now come down in price to levels that most G.A. pilots (even renter pilots) can afford. We review three of them: the Icarus AltAlert ($995), the AirSport ($599 to $899), and the CMM Altitude Tracking Module ($249.95).