Capistrano is a popular open-source tool primarily used for remote server automation and deployment of web applications. It simplifies the process of managing one or multiple servers by executing commands in parallel on all of them. Initially geared towards applications written in Ruby, Capistrano can now be used to deploy applications written in any language or framework by running arbitrary commands. This flexibility makes it an indispensable tool for developers who need to deploy applications and perform server tasks consistently and efficiently.