Free software to predict PKa values? - (Jan/31/2013 )

Hi

I was wondering if anyone could recommend some ideally free software to predict PKa values of some drug candidates? I just want to get a rough idea of what they are. Have been trying to do it using ChemBioDraw Ultra 13 but it won't work for my compounds.

Thanks

There's a lot of software in the web, but it depends on the molecule class, i.e. proteins, organic or inorganic acids/bases, phenols or whatever
http://www.moldiscovery.com/soft_moka.php
http://propka.ki.ku.dk/
http://www.acdlabs.com/products/percepta/predictors/pka/
http://www.chemaxon.com/marvin/help/calculations/pKa.html