I now made a dedicated CGenFF download page. It gets updated more regularly than the CGenFF section on the MacKerell lab's force field page and also contains a bit more information about the CGenFF version you're downloading.

Also, CGenFF version 2b4 is out. It can be downloaded at the CGenFF download page (the CGenFF section on the MacKerell lab's force field page has not been updated yet at the time of this posting) and will also be included in the upcoming c36a4 developmental CHARMM version. Here is a cumulative list of changes since CGenFF version 2a5 (the version on which the CGenFF paper was based):
  • Introduced carbamate parameters
  • Introduced parameters for extended ketones (contributed by yapolyak following a discussion on this forum).
  • Added limited thiocarbonyl support
  • Added parameters for phenylhydrazine
  • Added various other model compounds and parametes
  • Updated bonded parameters for -CH2-NH2(+)-CH2 group (somewhat developmental; future tweaks likely)
  • Corrected charges in RESI PPI2 topology
  • Slight tweak in bonded parameters for benzenes with aliphatic substituents
  • Slight tweak in benzamide bonded parameters
  • Slight tweak in bonded parameters for sterane scaffold and beta-ionone ring
  • Removed slack parameters containing NG2S0
  • Fixed atom typing mistakes in cholesterol. Doing so mandated removing and adding a few parameters.
  • Updated OCO angle in RESI DIOL.
  • Corrected atom types on RESI GABD.

Lastly, the CGenFF paper is finally available in print. The citation is:
K. Vanommeslaeghe, E. Hatcher, C. Acharya, S. Kundu, S. Zhong, J. Shim, E. Darian, O. Guvench, P. Lopes, I. Vorobyov, A. D. MacKerell Jr., J. Comput. Chem. 2010, 31, 671-690.