www.phantomfrog.com
I dont get money from posting that, lol.
But I so use them.
With phantom frog even if all your passwords were shared everyone would get blocked so no one that shouldn't have access would get in. And with the automated password recovery the real member can easily get a new password sent to their email instantly so they can log on to your site.
This means you wouldn't have to change the password for 400 users, and they wouldn't have to wait more then a few seconds to finish beating off.