I am looking for forum engine (for discussions) with good attack countermeasures built in. Windows (preferrably) or Linux. Free (as beer).

I think about registration flooding and blocking user accounts attacks.

For registration, such engine should have at least:

• captcha
• blocking mulitple registrations from the same IP
• providing login (for logging in) and user name (for displaying the author of the posts)

For logging in:

• no blocking on multiple tries -- instead after X try sending via mail a token, the third piece needed for next login -- without it logging in will be impossible (it would be similar to activation process)

The engine should be designed with two ideas in mind:

• protecting engine against attacks
• 0 penalty for decent users

Thank you in advance for your help and recommendations.