Simon Prosser

How it works.
The plugin monitors failed login attempts, if the limit is reached the users IP is added to a local banlist. 
The user is now blocked and can no longer attempt to login.
Every 24 hours the plugin will upload the blocker IPs and download a fresh list of all IPs blocked accross the network in the last 7 days.
If an IP is blocked on 3 or more servers its added the the global ban list and will be blocked on all servers using the plugin.

WP-DenyHosts

Mitch

Guard protects your wp-admin against bruteforce attacks.
Features:
<ul>
<li>Max retries: You have the power to choose how many attempts a user can make before the user is blocked;</li>
<li>Lockdown: This prevents the user from being able to use the login form after too many retries;</li>
<li>Email notifications: We’ll report a user lockdown to you via email;</li>
<li>Hide lost password: Hides the “Forgot your password?” link from the login form page.</li>
</ul>
This plugin does not make any changes to core: When you delete/uninstall the plugin, everything will revert back to normal.

Guard

Guard protects your wp-admin against bruteforce attacks.

WP-DenyHosts vs Guard