Princess is the Flash animated webseries Matt and Trey were asked to make for Shockwave.com in 1999. They were supposed to create 39 (I've also seen it reported as 48) episodes, but they were given complete creative control, so of course they were vulgar enough with it to get the series cancelled after only two episodes. These episodes were unreleased until Trio.TV made a documentary about Princess in 2004 and shared the episodes online. "Shocked" is a great watch! It features Matt and Trey talking about how intertwined South Park and the Internet are, which, as you might be able to tell from this site, is one of my biggest interests.
Be aware these episodes are sexually explicit and pretty stupid. But still funny.