Sabre is the very first plugin of its kind ever released for WordPress !
Sabre is the acronym for Simple Anti Bot Registration Engine.
Sabre is dedicated to the protection of your blog against unwanted registration by spam robots when you give your readers the possibility to register freely.
This modular protection includes a captcha or math test in the registration form. You can choose to include both or one test or let the plugin choose randomly the test.
Another possibility is to ask the user to confirm his registration by clicking on a link submitted by mail after he registered. This confirmation must take place within a maximum period of time or the account will become unusable.
You can have more informations and download it on WordPress.org at http://wordpress.org/extend/plugins/sabre/.