I've just started using
Better WP Security and I like it a lot, bans users temporarily for a myriad of different techniques people use to compromise your site.
I've also got
Bullet Proof installed on a couple of sites, also good.
The nice thing about Better WP is you can choose your level of security, some tweaks require a more significant server load than others, but I've been quite happy with it since I've installed it.