A fun project in Python. Think it is far from perfect (maybe better in Java or C++?). The less addresses, the faster it is, of course. It creates a random private key and checks if its address is in the list:
370k keys per second in average for 2256 possible combinations.
If god had started that program on my computer during the big bang, it would've tested 0.00000000000000000000062% of all possible keys until today.