The Ultimaker is a 3D printer kit sold from the Netherlands. It main points are printing speed and the size of the build envelope.
The build envelope measures 210x210x220mm, which is impressive given that the entire machine only is about 340x355x390mm. Compared to other reprap-derived 3D printers, it also uses an extruder head where the majority of the mass has been removed from the parts in motion. This makes it possible to print at higher speeds.
The Ultimaker is based on the reprap project, and is thus open source hardware. This means that as soon as production units start shipping, all the plans and software will be released. Still, buying a complete kit is probably easier, and potentially cheaper, than sourcing all the components yourself.
For more information about the Ultimaker, please visit the project wiki, or the Ultimaker page on the reprap wiki.