It's what alternating caps is officially known by. Alternating caps LoOk LiKe ThIs and tend to be used mostly in forums, because it takes quite a bit of time to type them out. I'm not sure why people do this, but they must think they've got a pretty good reason, because it's still being used despite the fact that it's very annoying.
Person A: "Hey man, check out my troutspeak. IsN't ThIs CoOl?"
Person B: "I think it makes you look like an idiot, actually."