Any free alternatives to MAPS?

I have not gotten an answer to what defines a user from MAPS. 

If the cost is $50 per 1,000 user and that means someone subscribed to a mailing
list, then Real Time will have no choice but to allow spammers to abuse our
mailing list server. It's just too expensive for us to pay for it.
-- 
Bob Tanner <tanner at real-time.com>       | Phone : (952)943-8700
http://www.mn-linux.org                 | Fax   : (952)943-8500
Key fingerprint =  6C E9 51 4F D5 3E 4C 66 62 A9 10 E5 35 85 39 D9