SpeckyBoy lists down the Top 10 Security and Protection plugins for WordPress. Though this post was written for WordPress 2.5 in mind, it still holds good for WordPress 2.6 as well.
With every site you build there are going to be security risks and issues, there is no way around this, it is going to happen. All we can do is minimize the damage, be ready for it and take action. WordPress is now the most popular Blogging Engine, this will not mean an increase attacks, but it will have the highest potential of attacks. I haven’t included Akismet, because I am sure everyone has heard about it.