Is there an easy way in WSS to track when files are downloaded from a site?
Yes see How to configure Usage Analysis Processing on a Web server by using SharePoint Portal Server 2003 and Windows SharePoint Services or by using SharePoint Server 2007 and Windows SharePoint Services 3.0
You can also get this data by creating a sharepoint designer report (office online appears to be dead or I'd post the link)
I believe you can accomplish this using the audit log reports that come with SharePoint. You need to turn on the audit logs, but check out this article for a bit more.
Basically once turned on, any action in your site collection will be audited, and downloaded files will I believe show up as a "View" action. The audit logs are kind of ugly and are XML based, Excel 2007 handles them nicely though. Hope this helps