As found on Wikipedia:
This is a minor revision from March 2005. The power was slightly increased on all rails. Efficiency requirements changed. Added 6-pin connector for PCI-E graphics cards, that aids the PCI-E slot in the motherboard, delivering 75 watts.
Another minor revision. Added 8-pin connector for PCI-E graphics cards, that delivers another 150 watts.
The most recent revision, effective March 2007. Efficiency recommendations were increased to 80% (with at least 70% efficiency required), and the 12 V load requirement was lowered. Higher efficiency generally results in less power consumption (and less waste heat), and the 80% recommendation brings supplies in line with new Energy Star 4.0 mandates. The reduced load requirement allows compatibility with processors that draw very little power during startup.