I know a guy that is working on "Web-Shield". It is a project from network associates that scans all incoming internet traffic. Very hardcore, the big one does something like 500 e-mails per minute so it in no way slows down your internet connection. It is a easy but not so cheap alternative. Aimed at the very big corporations.
Probably better than the linux alternative, but if your IT department does not have funding, well then I guess the alternative is for you...!!
Many tuts and stuff out there on how to make your own one based on a linux box.
Looks like someone just went out there and made a package for linux and made everyones lives that much easier...!!