The client application previously known as the Let’s Encrypt client is now Certbot.
An announcement with more information about the new project name is posted at https://www.eff.org/deeplinks/2016/05/announcing-certbot-new-tls-robot
You can get Certbot from https://certbot.eff.org/; we’ll be updating documentation in various places to reflect the name change. If you see old documentation that refers to the Let’s Encrypt client or the official Let’s Encrypt client, please feel free to suggest that the authors update it (or let someone here know if you see it on EFF or ISRG sites or in the Certbot GitHub repository).
A lot of people did a lot of work on the rename to make things smooth and clear. In most cases, there will be transitional packages and redirects, and so old filenames and links will continue to work properly (for example, it’s still possible to run letsencrypt-auto in lieu of the new certbot-auto, in case you have scripts that rely on the original name).