eMule

If you were using a computer in the 2000s then you have probably heard of eMule (it was developed as an alternative to eDonkey, which was the popular P2P sharing client back in the days). eMule is a P2P (peer-to-peer) client, which is still used by many people around the world. The program is completely free to use, but it is not updated anymore (it didn't receive any updates from 2010).