Since this post is in the Digital Performance section, I'm assuming that you're planning on dropping some acappelas in a live set?
One of the only free automatic pitch correction plugins I've seen is GSnap:
http://www.gvst.co.uk/gsnap.htm
Another option would be to use the regular pitch shifting available in most audio editors and DAW's on individual words or phrases to get them clost to the correct key, then save the overall vocal file and re-import it in your live performance sequencer.