One example is an estimate. Right now I have my project managers attach it to the project object in both Clarizen and SFDC. It would be nice if they could just attach it in Clarizen and have it show up in SFDC -- even if it was a link that took the user to the document (although, our salespeople do not have Clarizen logins).
An example going in the other direction (from SFDC to Clarizen) would be the Statement of Work document. The sales guy posts this in SFDC, but it would be nice if it went into the Clarizen project as well.