Prusa comes from Josep Prusa. He is a Czech engineer who invented the Prusa Mendel, Prusa i2 and Prusa i3. What is noteworthy about him is that his inventions are truly open source and kits of them are sold by many online vendors. He really stands by this and was very disappointed and vocal about it when Makerbot went closed source.
Josep Prusa is an awesome guy. I truely think Prusa is a company that will be around for a long time. I have a buddy with the i3, and it is fabulous!