Its called the 3Doodler.It "writes" on things and fresh air!You can build with it.It uses ABS plastic.A thin rod of ABS is fed through the barrel of the pen and heated to melting point.A cooling fan near the nib dries the ABS instantly.Watch the video on the link below.You can fill gaps, draw thin lines of ABS plastic.It costs $75.Looks like a really useful tool.
http://hexus.net/tech/news/peripherals/51861-3doodler-75-3d-pen-hits-kickstarter/