What is the best anti spam filter for Exchange 2003?

We use XWall. I have not used other filters, so I have nothing to compare it to, but Xwall is pretty easy to use and it’s robust.

Are there any free ones out there?

Exchange Intelligent Message Filter (IMS) Comes with SP2, multiple blacklists, Free

and TrendMicro Scan Mail for Exchange (Not Free).

IMS is catching 80–85% on its own, which amounts to roughly 80,000 to 100,000 per day for close to 8000 users.

Probably the best known free one is SpamAssassin

But this is not just for Exchange, it works on the SMTP e-mail before it reaches Exchange.