NovaCTF is a freeware for 3D-CTF correction for electron microscopy, 
as described in this publication:

Turoňová, B., Schur, F.K.M, Wan, W. and Briggs, J.A.G. 
Efficient 3D-CTF correction for cryo-electron tomography using NovaCTF
 improves subtomogram averaging resolution to 3.4 Å.
(doi.org/10.1016/j.jsb.2017.07.007)

The source code for NovaCTF is distributed under an GPL v.3 license.
