What are some uses of a signed crytocurrency message in a product or service or some other useful use of this feature? A signed message is a way to prove a user owns an address. But what are some useful products or services that have been built around this feature with signing a bitcoin / altcoin message with their private key?
This isn't necessarily a product or service, but Gigabyte distribution (Stellar Lumens distribution was done similarly) was done by distributing the currency to those who owned Bitcoin, based on the amount they owned. Ownership of a Bitcoin address was proven by either sending a micropayment, or signing a message using the address.