If anonymous access isn't acceptable to you, your best best would be
to upgrade to MOSS 2007 (or just WSSv3). MOSS supports forms based
authentication (FBA). SPS does as well, however it takes significant
effort to get it configured properly (I've never done it myself, I've
just heard horror stories). If you setup FBA, you could grant your
external users that are stored, say in a SQL database, access to any
site within the web application that FBA is configured on. It's
pretty painless to setup in SharePoint v3.