In Caesar's time, as far as I know, the capite censi voted - it was about their only right since they were so poor they didn't own anything.
Freedmen were enrolled in the four urban tribes and could vote, and there were several attempts to let them join the rustic tribes where their votes would have more weight (Millar, The Crowd in Rome in the late Republic, 1998, pg.35). I suppose they also voted in the comitia centuriata, but at present am not sure where to find the references for that - or for the capite censi. I promise to look for them when I have more time.