Webistrano is a Web UI for managing Capistrano based deployments (primarily on Ruby on Rails).
Webistrnao lets you define deployment steps and manage assets in "stages" like QA, Staging, Production etc. It enables tracking what, where, who and how an application is deployed.
Since each deployment is managed via an SSH session to the target machine, the only requirement is the ability to ssh to the target machine. No agent is required on the target machine.