The first Xbox had a small 50mm or 60mm fan running at 5v. It was so the console wouldn't sound like a jet turbine.
The Xbox 360 has a better cooling feature, but this is because the hardware runs so much hotter than the original Xbox's hardware did. The original Xbox had two small aluminum heatsinks covering the CPU and GPU, but the Xbox 360 has a slightly bigger aluminum heatsink for the Xenos GPU and a aluminum finned heatsink with a copper base and heatpipes. Two 80mm-ish fans move the hot air out of the case.
Recently, some European Xbox 360's were sighted with an extra heatsink attached behind the DVD drive with a heatpipe or two leading to the CPU heatsink for additional cooling. This is probably one of the features Microsoft has been taking to prevent the 'RRoD' or Red Ring of Death.