terms of use at login The user has to agree to a terms of use at EACH login, not just at registration. 
What is the easiest way to do this? 

Right now, I have a login page with only a login block on it (asking for username and password); once logged in the users are sent to the homepage. I need a pop up with terms of usage to appear after the login. The user accepts the 'terms of use' and are then directed to homepage, or are logged out.

There are several good modules that do what I want but only at registration, not for EACH login.
I'd check out Login Toboggan.  There's an example on the project homepage that nearly matches your use case.