reCAPTCHA an anti-spam tool for obscuring your email address on the web. And by using it, you're helping to 'proofread' for a project which is digitising books for the Internet Archive (I'm a big fan of IA because it hosts the Wayback Machine).
reCAPTCHA Mailhide helps protect your inbox from spambots by asking people to solve a reCAPTCHA before viewing your email address. It displays two words, and the user needs to type both words in to display your email address.
To put the reCAPTCHA tool on your website you can use an application plugin, a library for your programming language or an API - browse their list of resources for further info. It has plugins for Movable Type, WordPress and many other popular publishing apps.