A gravity powered aircraft is being developed by former govornment and
millitary scientists.  It works in similar to a helium balloon, except
it is also able to contain large spaces of vacuum which is much
lighter than helium.  The ultralight aircraft becomes lighter than air
and rises high enough that it can then release some of its stored
vacuum and begin gliding back down to earth like a conventional
ultralight.  Once this technology is developed, which may not happen
in our life time, I am sure it will become more advanced and
vacuum-engineering will be able to take us to space.  You can move
through space in theory the same way because there are a small number
of particles still in the void, you just need to create better quality
vacuums.  This technology works similar to a submarine which changes
this way underwater.  Doing this underwater is an easy way to show a
proof of concept with this technology, the only differance being that
the pressure underwater is greater so you don't need helium or vacuum
to make the craft rise to the surface.  Here is a good link for you to
enjoy on this gravity powered craft, lend your support!:
http://www.machinedesign.com/ASP/viewSelectedArticle.asp?strArticleId=56635&strSite=MDSite

and directly to the creators website:
http://www.fuellessflight.com/