For a password protected site, is there a way to allow users to log in to the site multiple times as long as it's on the same computer (even in different browsers)?
One way I have thought to do it is always send MAC address on login, but that's not available in browsers. Is there some other way of doing this?
Use a flash (or java, or ...) applet to get the needed unique information.
As I understand it, you want to keep your users logged in to your application even if they close the browser and switch to another one. Briefly looking into this, a possible solution might be to use "flash cookies" or local shared objects, which are managed browser independent. However, there are some security, privacy and legal concerns plus the drawbacks from using the proprietary Flash technology. I would go with Alex's advice of rethinking your approach.