Registration Reminder is a fantastic component, that performs exactly as described. I have used this on a association website with ~1000 registered users, many of whom have never logged in. We had a 25% response rate from a single email!
Dwayne (the developer) was amazingly prompt and helpful in his replies, even when I asked some obvious questions!
I would highly recommend this component if you have a user base that is apathetic in using your website.