I think it would be quite useful to be able to link a username on a particular website to an Ethereum Address. This would mean that a user of a forum or website like reddit, forum.ethereum.org, facebook, twitter etc could prove that they are involved in a particular contract based on their username on a website.
A simple way to do it would be to create a registry contract which stores a set of domain and account names for each address. Optionally, it could also store a permalink to a post by the user announcing their address, so that an address owner can't link to arbitrary usernames they don't own.
I realise the security of this approach is not super high since the account on the site can be compromised by the site owner, or due to a hack. But it might be useful for people to have information about them publicly available and cross-website. For example, you could imagine a website plug-in for displaying a user's balance of a given token. The website wouldn't have to additionally store the user's public address and could access the registry with just its domain name and the user's username.
I'm wondering if someone has done this before so I don't re-invent the wheel. Or if maybe there is already a better approach?