I'm looking to compile a list of altcoins with plaintext transaction messaging embedded into the blockchain. If anyone knows of a coin that fits the bill then please share.
check out: https://nxtwiki.org/wiki/Arbitrary_Messages
The NXT Messaging system allows you to send and receive data on the NXT Blockchain, thus allowing any Nxt account holder to communicate directly with all other account holders.
Messages can be attached to any Nxt transaction, making bookkeeping easy, as you can tag all your transactions with a description. Project developers can use the Messaging system to embed machine-readable data within an Nxt transaction. This allows automated functions by reading the data sent to you on the blockchain.
Messaging is securely encrypted by default, but text or data can also be sent unencrypted.
Ethereum does not have a messaging system by default but, because it is programmable and you can store arbitrary data in the blockchain (more exactly, in a Merkle tree whose root is in the blockchain), you can develop it fairly easily.
It is actually a very common "my first Ethereum program", just after "hello, world" : http://jonathanpatrick.me/blog/ethereum-smtp https://ethereum.stackexchange.com/questions/1205/how-can-ethereum-be-used-to-send-messages-between-connected-machines https://github.com/ethereum/wiki/wiki/Whisper and many many others