Akismet does a pretty good job of catching most comment spam on WordPress blogs, but you still tend to end up with hundreds (in my case thousands) of user accounts registered by spam bots that just sit there cluttering up the user list.
User Spam Remover is a plugin for WordPress that fixes this problem by automatically removing any accounts that have not made a non-spam comment in > x days since it was created. It can also be set to block that stupid email notification that WordPress sends every time a new user signs up.