Let’s Encrypt is a free, automated, and open certificate authority (CA), run for the public’s benefit. Let’s Encrypt is a service provided by the Internet Security Research Group (ISRG).
The service is due to launch in summer 2015, but unless all browser vendors choose to include it as a trusted CA, it will be dead in the water. So far, I haven’t been able to find out if such an agreement has been made. Maybe they are using IdenTrust as a cross-signing partner.