(1) http://www.ollydbg.de/ : Old school favorite (32 bit) - What we saw in the class
(2) https://www.immunityinc.com/products/debugger/ : A customized version of OllyDbg -- You can use python for debugging (setting up breakpoints, changing binary code, and so on)
(3) https://x64dbg.com/ : Open source 64bit debugger