Once I had to patch a command line executable with a help of OllyDbg. Had to remember university lessons of assembler.
Just a couple of jumps were modified and the program started behave as desired. OllyDbg is a very nice tool. Small, smart and powerful. I spent approximately 8 hours on a task.
Would save couple of hours if didn't try to use Visual Studio for that purpose and started to use OllyDbg from the very beginning.