Well, the first one that wasn't an exercise from Kernighan and Ritchie, anyway. I have made an implementation of the Sieve of Erastothenes, which is a way of finding prime numbers. (Wikipedia has a good explanation, and an animation!)
I am rather proud of myself. The source code is here and the executable is here. If you run it at the command line, type in a number and then press enter, it will tell you all the prime numbers that are smaller than your number.
My next post will be more about hearts, unless I write any cool programs before then.