Localhost

See 127.0.0.1 -- it is an IP address for the computer you are currently using.

My favorite alias for localhost is paranoia.stardot.com. I often use postmaster@paranoia.stardot.com or abuse@paranoia.stardot.com as my e-mail address when giving bogus registration information.

Many sites have good aliases for localhost, such as warez.slashdot.org.

As a correction to pb's note above, localhost is not Unix specific; it is a standard DNS convention, and is thus present on all types of machines that have a DNS client (kind of a silly notion NOT to have one). Of the commercially popular platforms, this also includes Windows (any version with a valid Winsock) and Macintosh. They will all reverse lookup to be 127.0.0.1.