Function Grapher 2 is a wonderful little app that graphs mathematical functions. It's extremely simple, but complex enough to be able to graph terribly difficult equations. You can plot almost any function you can type in, then plot its derivative, evaluate the function at some point, evaluate the derivative at some point, and even do integration over a period. You can also graph implicit functions, zoom in and out, and save to a file (which can then be opened with the click of a button, if you're working with a certain graph over an extended period of time). The last thing to mention is that it's very much like a workplace, in that you can have multiple graphs open and switch between them.
Function Grapher is very small, only 56kb, and runs at next to no resources. I'm somewhat convinced that it is not portable, since my system now has the FGR extension (the saved files from Function Grapher) associated with it, and I don't ever remember asking it to do that. So while it may or may not require any files outside of itself, it will leave traces in the registry.
It's a handy little app for students. The only feature I wish it had would be like Wzgrapher, where you can save the graph as an image. But otherwise, it could be a life saver in those Calculus courses.
Visit Function Grapher 2 (Madhavan's) website for download