Squad just released a new free demo based on an 0.18.3 Unity 4 build. This should really blow you out of the atmosphere!

* Part loading limited to the 25 stock ones.
* No Plugin loading.
* SPH and Runway Facilities are inaccessible.
* Only one save profile allowed.
* Cannot launch new flights if 3 or more are already in progress.
* Kerbin, the Mun and the Sun are the only celestial bodies in the solar system.