It's all proprietary, I'm afraid, and interoperability is pretty much in-brand when it's there at all. (Though you may expect third-party makers to a least try compatibility with Canon's RF system at some point.) Some pro-level equipment is compatible with the PocketWizard system, or at least versions of it, but that seems to be licensed. (I'm sure the good folks at PocketWizard have nothing against convenience, but they'd probably just as soon you bought another of their units rather than simply copy their signalling system. Oh, and you wouldn't be able to say you're "PocketWizard compatible" without violating their trademarks either.)

As for the cheaper RF triggers/receivers, the compatibility is pretty much within a model range. Cactus V5 triggers, for instance, are not compatible with V4s, which are not compatible with V3s, and so on. It's the same situation with Yongnuo and Phottix -- unless they tell you that different units are compatible (usually with a common "name" for the range of devices), they won't be. Frequencies vary (newer units tend to work in the 2.4GHz band, while older ones are in the 900MHz range), as do signal encoding schemes and so forth. At the low end of the market, there's no real requirement to protect the users' investment in your old technology.