Is there a way in WSS 3.0 to send an email notification when a list attachment is downloaded or when a document in a document library is downloaded?
The event listeners you can attach to list items in WSS only apply to add, update and delete events.
You'd have to map an HTTPHandler to the URL of the list/library storing the content.
This handler would be responsible for sending notifications when it's detected that a client is making a request for an attachment or document.
Another possibility is to leverage SharePoint Auditing. SharePoint auditing can log whenever someone accesses an item. If you had a process monitoring and querying the SharePoint Audit Log you could fire off your alerts.