In general, anything that is great, helpful, or unexpectedly awesome.
Nifty can be used in many different contexts.
Nifty is a great describing word and will soon be used by the entire population. Wouldnt that be nifty?
My, isn't that a nifty pen?
Wow! look at what my niftypen can do!
She probalby has a lot of nifty knowledge rattling around in her ol' noggin!
I printed off the page for you as it has some nifty studying tips on it.
take a trip to see your math teacher; they love nifty.