What are some open-source all-electron DFT alternatives to Wien2K?

CP2K

This program can do all-electron calculations using the GAPW method.

Questaal

• Questaal is a suite of electronic structure programs. The codes can be used to model arbitrary materials, but they are mostly designed to answer condensed-matter theory questions about solid-state (periodic) structures. The majority of the codes use an all-electron implementation of density-functional theory. This includes several forms (Hamiltonian and Green’s function) that serve different purposes. There is an all-electron implementation of GW theory, used most particularly in a quasiparticle self-consistent form (QSGW). Tight-binding based on user-supplied empirical Hamiltonians is also supported. Recent development work includes Dynamic Mean Field Theory (DMFT-QSGW), the Bethe Salpeter Equation (BSE), and direct (Green Function) solution of the Dirac equation.

• Website: https://www.questaal.org/

Elk / EXCITING

Like @Nike Dattani suggested, Elk is an open-source well-documented FP-LAPW DFT package. Also, LAPW stands for Linearly-Augmented Plane Wave. Singh's book is an excellent introduction for this topic. (Disclaimer: I'm currently developing an extension to Elk / EXCITING to compute response functions and estimate Hubbard model parameters using the constrained RPA method, here in our research group at U. of Tennessee. Coincidentally the "working" name of the package is Exciting-Plus, but it's still being optimized so not ready for public use yet.)

Elk

• Free, and can be redistributed or modified under the terms of GPU General Public Licesence.
• Well explained on this webpage.
• Well documented in this PDF file.
• Interacts with the following codes:
  • Wannier90
  • Phonopy
  • NOMAD
  • Libxc
  • ASE
  • DGrid
  • Elk optics Analyzer
  • PyProcar
• Advertised in a few places as being focused strongly on being simple and easy to develop.
• Written in FORTRAN 2003 and an advertised strength is that it's all written in one language.

Finally, you said that you were particularly interested in codes that invite developers, and this exactly the case with Elk "Groups and individuals are actively encouraged to develop and release their own specialised versions of the Elk code under the GPL". The Wannier90 interface mentioned above, was developed by people in Sweden and Russia, who were not part of the original Elk team.