Mastercoin is an open source project driven by the members of the Bitcoin community that want to see more advanced features implemented on top of the blockchain.
Mastercoin enables the Bitcoin eco-system to become more decentralized and operate more in line with its peer to peer nature. History has shown that when a digital service has a single point of failure it will be attacked and taken down. Mastercoin removes many of today's bottle necks, that are preventing the wide spread use of Bitcoin.
Web wallet
http://www.omniwallet.org
Transaction explorer
http://masterchest.info
Reddit
http://www.reddit.com/r/mastercoin
Mastercoin specification
https://github.com/mastercoin-MSC/spec
Master Protocol Education
http://masterprotocoleducation.org
Mastercoin Wiki & FAQ
http://wiki.mastercoin.org/index.php?title=FAQ
You may also like to check out Mastercoin on Linkedin, Google+, Twitter or the Mastercoin group on Facebook. :)