Command-line calculators, that takes me back to when I first started programming in C++. I'd always use them for my math classes and they would even "show the work" i.e. the steps needed to solve the problems. I never felt bad about using them for homework 
I mean, if I could make a program do it, that meant I knew the subject material pretty well.
I mean, if I could make a program do it, that meant I knew the subject material pretty well.