RFC4387 is a standard for distributing certificates and keys over HTTP(s). Applications are Email clients that can automatically fetch other people's certificates to use them for encrypting emails. RFC4387 is the preferred mechanism for Internet-wide certificate publishing. (LDAP and HKP are good for local networks, but have their drawbacks on the Internet)

Implementation status
Crypto Platforms:
||Cryptlib||http://www.cs.auckland.ac.nz/~pgut001/cryptlib/||Supported||

CAs:
||CAcert||to be implemented soon||

Client applications:
||Thunderbird|| ||
||Outlook|| ||
||KMail|| ||