Nice resource from Physicians for Human Rights
Despite the fact that their title makes me wonder whether there is a group of evil Physicians against Human Rights, the PHR have done a nice series of fact sheets on various crowd control devices here. There are fact sheets on acoustic weapons, directed energy devices, rubber bullets, stun grenades, water cannon and, most pertinently, tear gas (and other chemical irritants).
A useful little primer even if it doesn't cover some of the next generation of riot control agents. Thanks to the Pandora Report for promoting it.