In 2009 a solution to a known problem known as "Double spending" was presented. The first implementation based on proof of work was introduced back then.
I didn't like that very much as I had began to like how the Ripple Monetary System works. The new solution could of course be used as part of Ripple but I didn't really like that idea as I was not sure if the new solution really worked or if it was some sort of scam which lots of people blindly believed in was not a scam.
The problem though is that Ripple exists only in theory and in a proof of concept at https://classic.ripplepay.com/. A real decentralised solution is still in progress as of 2013. I hope to use and get introduced to a working copy of Ripple one day.
Though as days went by and I didn't understand the system that tackled double spending, I got to know that the only 2 technologies that were part of it were Proof of Work and Public-key Cryptography with only the digital signatures part included. The proofs of work being shared between computers when a solution was found.
When I finally understood this system, well, it's just like that. When you understand something there's a special feeling. This system may have its little flaws but unless PKC or PoW are found to be broken this system will stay stable.
checksums, proof of work, digital signatures in PKC, public-key cryptography