A professor accidentally sent me a copy of my letter of recommendation for grad school. What should I do? I am applying for Masters programs in the USA. Most programs require the applicant to waive their right to see their letters of recommendation, and the applicant is not supposed to ever see their LOR.

One of the profs who is writing letters for me accidentally sent me a copy of my LOR today. He was supposed to send me a copy of a letter granting me permission to use certain hardware in a lab, but accidentally sent me the wrong letter. I suspect that he had both saved in the same directory on his computer, and simply chose the wrong one. 

The issue is that he has already sent this letter to some colleges. What is the correct thing to do in this situation?
You still need that hardware letter so I guess some uncomfortable situation may have to happen. Yet, I think some awkwardness can be avoided by focusing on the application. I'd suggest e-mailing him and tell him that it appears a wrong attachment was sent to you, and invite him to double check if the hardware approval letter was uploaded to the school application portal, follow by a polite ask for the actual approval document. End with a statement saying that you'll delete his previous e-mail and the attachment, and thank him for his help.

If you have not read it, then delete it. That'd be the best scenario. If you have read part of it by accident, notice that you have done nothing wrong. You simply waived the right to read it; you did not signed a contract to never read it. Yet, don't try to read all of it even it's not your fault, it isn't meant to be for you to read, and knowing the contents may affect you, even you think you can be completely objective.