Is there a way to approve or reject workflow activity over the email instead of logging into the CME?
In the email for controlling the workflow decision activity you can add two links to an ASP.NET page. When loaded ASP.NET page can access Tridion with Core Service and do the action you need with the workflow.
Links need to have some information in query string like is it approve or reject, activity tcmuri. This information must be provided when the email is created. Approve and reject info is used to determine which next activity will be activated by Core Service
Yes as far I remember there is Microsoft Outlook connector for SDL Tridion 2013 to expose workflow activities to Outlook tasks though I haven't tried yet. Please check SDL Tridion 2013 installer dump for same.
You may perform tasks like Approval, Review, Reassign etc without entering SDL Tridion CME.
Try this one as well, I am not sure if these are same.