The Notify dapp lets you send a notification over Ethereum using "pseudo-tokens"; any wallet that supports ERC20 tokens is capable of receiving Notify messages!
We use a very small smart contract that implements just enough of the ERC20 interface to allow wallets to interpret them as ERC20 token contracts.
When you send a message, one of these contracts gets created with your message and a transfer event is emitted that the recipient's wallet will interpret as receiving a token. When their wallet looks up the name of the token they just received (part of the ERC20 interface), the contract returns your message!
To use the Dapp, just paste the address of the person you want to notify, type in a message and hit Notify! The next time they look at their wallet, they'll see your message; it's that easy!
Once a message is received, the recipient can use their wallet to "transfer" the token to any address. When they do, the contract simply deletes itself, as it has served its purpose. This keeps the recipient's wallet free of old messages and saves the Ethereum network a few bytes of storage.
A single notification only costs about 75k gas, as much as 3 eth transfers or about $0.02.