Here you can download the HYBRID multiple-site titration code described in M. K. Gilson, "Multiple-site titration and molecular modelling: Two Rapid Methods for Computing Energies and Forces for Ionizable Groups in Proteins", Proteins:Struct. Func. Gen.,15:266 - 282, 1993. Please note that the code does not compute any electrostatic energies or interactions. These are required as inputs in the data file.
HYBRID is usually very fast so long as the clusters of ionizable groups don't contain more than 10 or 12 groups each. The code forms the clusters automatically based upon the interaction energies provided in the input files. Groups that interact strongly are grouped into a cluster, while interactions between clusters are weak.
The code and its usage are described here. A sample input file, data file and the corresponding output file are also available. The demonstration files are for 14 histidines with some strong interactions, so the titration curves are "abnormal". (To download these files to your disk, right-click the hyperlinks and select "Save Link As..." or "Save Target As...".)
The code is freely available for academic or personal use. I do ask that you cite the reference above in publications that use the code or another implementation of the same method. If you wish to use it for a commercial application, please contact me.