Peercoin, originally called Peer-to-Peer Coin, was one of the early successful altcoins. It implemented some new technologies that are not present in Bitcoin. Many of these new technologies have been adopted by a large number of altcoins that came after Peercoin.
Peercoin was probably the first altcoin with a focus on energy efficiency. Anyone who understands today's bitcoin mining industry or has ever had to download the Bitcoin blockchain knows first hand how inefficient and wasteful the Bitcoin system is. Peercoin brought many advancements that reduced the size of the blockchain and introduced a new algorithm that required much less energy when mining.
Peercoin has a well established and mature community. It has been a solid investment ever since it was created in 2012.