I know that a few people mentioned this test in their responses as ways to figure out if something was prime, but I was reading a little bit about this one, and thought it was pretty cool that there is a way to figure out the probability that a number is prime. This still isn't a definite way to know if something is prime, but it is even closer than the flt, because it gives you an estimation of how likely it is that it is prime. It is based off the same concept as the flt, but you make it into a probability test - and you can pick how small you want the probability to be of a number to be composite (making it a larger probability that it's prime) when you are picking numbers.

Here's how to do the Rabin-Miller test:

http://www.maths.abdn.ac.uk/~igc/tch/mx3015/notes/node156.html

I also thought this was a cool connection between number theory and probability theory (which I know a lot of you have taken already!)