Windows and Application Based Authentication

I realize there may be several approaches to accomplishing this, but if anyone has some advice on the best and simplest method, I would appreciate it...

Would like to have an intranet page that captures a user's domain username automatically when they are on a domain computer. I know this can be accomplished by setting up IIS to use Windows Authentication and then capture the LOGON_USER variable.

The problem is I would also like to make the site available outside the domain network, using some kind of form-database-based login within my application. For this I would have to allow anonymous access.

I really would be ok allowing anonymous access for the whole site and controlling security at the appluication level, but then I wouldn't be able to get the username of a domain user.

Any thoughts or help would be appreciated.