The public part of PublicKeyEncryption.

Stored in SSH as a file often called 'identity.pub'. You download people's public key using gpg --recv-keys, or from their webpage, to verify that something signed with their PrivateKey really came from them, or to encrypt something so that only their private key can decode it.

Public keys are actually used in several different widely used programs. The place most people encounter them (though most of them are unaware of the fact) in in a WebBrowser when they traverse an https link. Other places are in PGP/GPG. There are several projects that even use a public key identifier as a routing destination in an InternetProtocol-like protocol. You can find the list on the CAKE Wiki list of other projects.