This is one of the great feature that we have developed in SharePoint for document signature. Which is compatible with WSS too.
Signup at DocuSign website for a free account and use this SharePoint feature available at CodePlex to sign documents without any cost.
This is a simple/basic Version 1.0 of the tool that lets you sign or send requests for signature from a SharePoint library.
Lot more can be done. Please try this feature and provided your feedback and suggestions in Codeplex.