Good free programs are Designspark, 123D and Creo Elements... or so I've heard. I don't use them personally (I do use Creo 2.0 Parametric at work, along with Geomagic and Catia, but apparently it is very different from Creo Elements). I don't that they can only save as native format and stl, you won't be able to export as stp, igs, x_t, etc.

Good luck and would love to hear feedback on which you chose, why and how it works for you!