Developer website: Judebear Software
MacSignify generates random signatures for email and other communications applications. It independently randomizes a URL and a signature, with various user definable options to control how your signature looks. Includes quite a few example signature files and documentation to help you make your own. It works with any application which can attach a signature from an external plain text file. (Eudora is just one example.) Version 3.0 adds many new options, including finer control over the display of the "now playing" info, and a new timer feature to generate a new signature automatically at user-defined intervals.
MacOS 8.0 or later, CarbonLib.