CP2K  is a freely available density functional theory (DFT) package, licensed under the GPL. The code is developed by a number of individuals and groups, and currently numbers over 50,000 lines of Fortran 95 code. Key developers include members of Prof. Jurg Hutter's group in the Physical Chemistry Institue of the University of Zurich, in particular Dr. VandeVondele who collaborated closely throughout the project.
The key difference between CP2K and other DFT codes is its implementation of the Quickstep algorithm, which uses a dual basis - atom-centred Gaussian functions to represent the wave-functions, and plane waves/regular grids for the electonic density. See the Quickstep paper  for full details of the algorithm. In practise this approach requires an efficient and scalable method for transforming between the two representations, which is the subject of most of the work in this project. Like most other DFT codes, CP2K also makes use of 3D Fast Fourier Transforms (FFTs) to convert from the real space to reciprocal space (plane wave) representations.
As CP2K is under constant development, the modifications made under this project were able to be committed to the main CVS repository directly after passing the regression test suite and are already available for users to download. A recent version of the code is installed as a `package' on HECToR and HPCx.