Free software for calculating the net charge of an amino acid as a function of pH?
I like MarvinSketch. You can run it in your browser or on your computer. Under Insert > Template, you can find all the amino acids. Once you've inserted one, select Calculations > Protonation > Isoelectric point and click OK. You'll get a window that looks like this showing the net charge versus pH (I picked a pretty boring one, but other amino acids produce more complex curves):