Free Hydro/Aero-Dynamic Software Simulators?

CottonCandyEater asks: "Every second geek dreams of owning a little plane or a yacht/sailing boat, but only few have the money to buy one. The web is full of links on how to build your plane or boat yourself. However, the right way to do it would be to simulate it with aero/hydrodynamic fluid simulation software first, in order to get performance estimates and to optimize the shape. Is there any such software available for free? maybe out of the academic world? I don't mind if it has a front-end or not, as long as it is free."