Login Lockdown by Michael VanDeMar protects WordPress against brute force or dictionary attacks.
Login LockDown records the IP address and timestamp of every failed login attempt. If more than a certain number of attempts are detected within a short period of time from the same IP range, then the login function is disabled for all requests from that range.
The plugin defaults to a 1 hour lock out of an IP block after 3 failed login attempts within 5 minutes. This can be modified via the Options panel. Admisitrators can release locked out IP ranges manually from the panel.
Plugin Homepage: http://www.bad-neighborhood.com/login-lockdown.html