Identities/PubKeys

Alternate form of authentication
Public/private key pairs (think PGP).
Stored in ~/.ssh directory by default.
Stored as two separate files, 'name', and 'name.pub'
Client offers to authenticate using each available key.
Server has copy of public key(s) and will accept key authentication if it's available.

User-based authentication mechanism, not host-wide.
Host-based authentication is separate possibility.
HostbasedAuthentication or RhostsRSAAuthentication configuration in sshd_config.