Jaime told Brienne that there are three men in the kingdom who might have a chance against him in battle. He was presumably referring to Gregor "the Mountain" Clegane, Sandor "the Hound" Clegane and Ser Barristan Selmy.
According to the novel, Robb was not angry at Catelyn at all for releasing Jaime. He did not have her guarded or any otherwise punished. On the contrary, he told her that he knew she did that out of love for Arya and Sansa, and out of grief for Bran and Rickon.
The trivia item below may give away important plot points.
According to the novel, Arya did not ask Jaqen H'ghar to kill Tywin Lannister. She named Jaqen as the third victim in order to force him to help her release the Northmen prisoners at Harrenhal. He agreed to help her if she named another name. Together with Rorge and Biter, they killed the guards and released Robett Glover and the Northmen prisoners. The freed prisoners joined forces with the Brave Companions, and turned Harrenhal to Roose Bolton.