Though I believe Sunny when he says that him and Scott Nadal invented it independently, if you're interested in the history of Proof of Stake you might also like
Bitcoin, Namecoin, and Peercoin are first attempts at solutions to important, long-standing problems. Bitcoin is the first decentralized cryptocurrency. Long before bitcoin existed, people believed that the problem of decentralized cryptocurrency was important.
Namecoin, as far as I know, is the first decentralized DNS system. Again, long before namecoin existed, people hoped for a decentralized DNS system.
Peercoin is the first decentralized cryptocurrency based on proof-of-stake. Peercoin's inspiration is the 'tragedy of the commons' associated with PoW mining. The 'tragedy of the commons' problem in mining has a history dating back to this thread in Nov 2010. People began searching for a solution before peercoin existed and before the term of proof-of-stake had been coined.
When I found out about bitcoin (March 2011), it sounded awesome to me. Proof of stake was not a term yet. In April 2011, I became aware of the the tragedy of the commons problem. I saw it as a nail in bitcoin's coffin and I still see it this way.
Discovering proof-of-stake got me excited again. The beauty of PoS is that it does not require miner rewards to function. With proof-of-stake, the tragedy of the commons doomsday scenario becomes a non-issue. Sure they may be no txn fees, but so what? If you are using PoS this is a great feature. If you are using PoW low fees are a showstopping bug. Once I learned about proof of stake, I saw possibilities: 1) Bitcoin adopts proof-of-stake (or some other rule-breaking alternative) and remains dominant in the long-term. 2) An altcoin adopts proof-of-stake and remains dominant in the long-term. Personally, I see exit to a new cryptocurrency as much more likely. The political challenges of rule-breaking changes in bitcoin are just too daunting. If you need to rewrite the rules, starting from scratch can be a valuable asset.
Today, few people understand that the proof-of-stake idea came out of the debate about the tragedy of the commons problem. If you haven't done so already, I recommend reading one of the most influential threads in the history of bitcoin talk: the tragedy of the commmons thread by Vandroiy. Vandroiy's thread attracted a lot of interest, but did not really make any progress towards a solution for two months. Things changed when Quantum Mechanic posted the thread "proof of stake instead of proof of work" (
https://bitcointalksearch.org/topic/proof-of-stake-instead-of-proof-of-work-27787). Quantum's thread is the first instance of the term 'proof of stake' in the bitcoin community. It may be the genesis of the concept as well. Quantum suggests that proof of stake could allow for lower txn fees. Quantum's concept of proof of stake starts to gain traction once it appears in Vandroiy's thread. Quickly, Vandroiy, Meni, myself, and others realize that proof-of-stake is the likely solution to the tragedy of the commons problem. From this point on, I make it a personal mission to promote proof-of-stake cryptocurrency.
To date, proof-of-stake remains the only proposed solution to the tragedy of the commons problem. With peercoin's arrival in Aug 2012, we have seen this solution realized. Bitcoin devs have spent three years searching or an alternative answer, but have yet to even propose anything.
The tragedy of the commons problem matters. Gavin has often said that a working market for txn fees is perhaps bitcoin's most pressing development issue. This is just a different (and less dramatic) way of referring to the tragedy of the commons problem. Game theory I learned in school told me that bitcoin has 0% chance of remaining valuable in the long-term unless the problem is fixed. If bitcoin can't be valuable in say... march 2050, then why would it be valuable in february 2050? Working backward to the present, the implication is that bitcoin's value depends on faith that this will get sorted out.
Freicoin and Primecoin also solve novel problems, so they deserve credit too. However, very few people see the motivating problems behind Freicoin and Primecoin as important. The tragedy of the commons problem, on the other hand, is one of bitcoin's most important lingering issues. For the past 2 1/2 years, I've viewed this problem as the number one threat to bitcoin's survival.
tl;dr Peercoin offers the first solution to one of bitcoin's most important problems. Can you tell a similar story about the intellectual contribution of litecoin, Quarkcoin, [Insert Scamcoin name here], etc.? Even if Peercoin gets replaced by something else and loses all value, it will still have made history. Nothing can change that. Very few coins can say the same.