Does anyone know of a FREE/Very Cheap Spam filter that I can use with Exchange 2003?

There's one built in, though I'm not sure how good it is. What we do is use a Barracuda Spam Firewall in front, but it's not cheap, sorry.

You could try to do Spamassassin, but that can be tricky. It does rock though.

If I was doing it on the cheap, I'd use a dedicated linux box with Spamassassin in front of exchange: I don't trust Exchange connected directly to the Internet.