Wow, so much ignorance in this thread!
First: It is 100% legal in the U.S., if you reasonably fear for someones safety, to stop an assault on them using a reasonable amount of force. Nobody goes to jail or gets successfully sued for that.
Second: Using more than the reasonably necessary amount force to stop an assault is illegal. It makes the person playing macho good Samaritan now guilty of assault. The case cited of the guy who found his girlfriend being sexual assaulted didn't just stop the assault, he then beat the guy up after he was no longer a threat. Force can only be used to stop the assault, not a second longer! If you decide to shoot an attacker to stop them, you better be able to convince a jury that it was the appropriate amount of force to use.
Third: Unless you are unable to control your temper, no person should be afraid of helping a woman being attacked. Citing that you would have done something but you were afraid of being sued makes you a douche, unless others were already rendering aid.
Forth: The case were the U.S. Supreme Court upheld the lawsuit against the Good Samaritan was because the woman trying to "rescue" her friend panicked, overreacted and did far more harm than good when she drug her from the car, which was NOT on fire. The court basically said that if you are rendering aid you had best do only what is reasonable and does not harm the victim any further.
Conclusion: There is no law in the U.S. that forbids using reasonable force to protect the safety of you or another person. The key is you have to be reacting to a creditable threat and only use a reasonable amount of force in response. All you guys stating otherwise are trying to champion incidents where too much force was used in response. You can't legally beat a guy half to death, even if you catch him raping your girlfriend. All you get to do is stop him. Unless you are in Canada, then I don't know what you get to legally do.