" HiddenID is a proposed standard for Tor users to be authenticated and [in the future] peers and service providers would be able to manage internet-wide reputation of such identities. Such a standard is needed because at the moment, most mainstream internet services treat Tor users like common criminals :)" 
Essentially, each user runs an OpenID server per account. There may be a need for multiple accounts "Because a gay communist cancer-patient job-seeker deserves at least 4 identities" , and one of the purposes of HiddenID is to minimize the risk of unification of those accounts (i.e. partial or ful deanonymization).
The only known HiddenID-friendly site so far is HiddenID Den, a wordpress blog using the HiddenID wordpress plugin. A HiddenID-friendly python-openid fork is also available (so far there are no servers using it "in the wild"). In both cases, tor must be running on the server (which might be a barrier for many site owners).