Underline is a variant of the Openline client, an open source hotline client for MacOS and Windows.
The primary difference between Underline and the Openline client is that Underline optionally uses blowfish to encrypt connections to servers that support it. Underline only supports encryption on the primary socket and not on sockets established for file transfers. At this time no known hotline client/server supports the encrytion of file transfers.
Underline (like Openline) is released under the GNU GPL. The source code can be obtained here.
MacOSX users should use the carbon version of Underline and MacOS9 users should use the classic version, both downloadable here.
The Windows version can be downloaded here.