Tired of spam registrations in your Joomla website? Joomla 2.5 version now has built in plugin to add recaptcha. Recaptcha is free service from Google which provides a user to enter the two words (These words can't be read by bots) before registering.
Step 1: Go to http://www.google.com/recaptcha. You need a Google Account to log in. Get a public key and private key for your website.
Step 2: Go to Plugin Manager in your Joomla backend and open the Captcha Recaptcha (You can filter by type) plugin. Enter both the keys generated in step 1 in parameters. Don't forget to enable the plugin and save.
Step 3: Check your registration page. You should see recaptcha by now. Try a dummy registration to see yourself how it works.