Usermin is the open-source end-user interface from the Webmin team, served on port 20000, managing email, files, and passwords without IMAP/POP3, with low resource usage for lightweight self-service panels.
Ajenti is an open-source Linux server management panel created in 2012 by Russian developer Eugene Pankov, MIT-licensed and free with 8k+ GitHub stars, lightweight and plugin-rich for visual administration on low-end VPS.
Box is an enterprise content cloud platform offering file management, collaboration, security compliance, and workflow automation, founded in 2005 by Aaron Levie and Dylan Smith.