SPAM!!!! is a very big problem for all big applications. If you are having a site which allows users to comment on ur posts and so on then you might get affected by spamming.
Akismet is something which is helping us to avoid spamming on our sites.
There is a rails plugin available at http://github.com/jfrench/rakismet/tree/master
You can download this and configure akismet in your rails app. more information is provided in read me file of the plugin.
IF you want to see the actual implementation you can watch a screencast on
Hope this is some useful information for you.