if its really minor a quick tweak with finger and thumb should be enough, had this happen a few times after rocks or sticks have hit a rotor (203mm poss a bit more prone!).
You can buy park tool specific rotor straightening tools but to my mind they arent even as good as an adjustable spanner for more severe bends (which also works well on dented rim sidewalls as the broad flat surfaces of the spanner doesnt mark the metal).
I thought they were a joke when I first saw them, best part of 20 quid for something so simple (I know someone with four…)
http://www.chainreactioncycles.com/Models.aspx?PartnerID=79&ModelID=12324