I have a .net web application that contain all my business logic but my blogs and other static pages are in sitefinity my authentication is in my web application but i want to authenticate them in sitefinity so they can comment etc. on blogs and news without login in. Is their a way to do this?
If I understand, you want people to log onto your Sitefinity site using the creditionals from your web application.
You can create a Custom User Provider and add that to your Sitefinity site. That will authenticate them. Though I think that will require them to Authenticate on each site.
If you want a Single Sign On effect where they can log on once at either site then you need to implement the Sitefinity STS site.