This very website you are reading right now is hosted on a platform that I helped to create when I was 21 years old. GoDaddy cPanel reached a million users a year later. I learned so much about REST provisioning APIs that talked to the rest of the company's outdated services, let me tell you! I couldn't have hoped for a better entry-level job. The team was really brilliant and I felt very supported.
LiquidWeb's Managed WordPress
This was a fun platform to build once I had a lot more experience and helped me to get my hands wet with some frontend. This was to help WordPress site managers manage everything they would in wp-admin through a click of a button in one place. It was fun to provision a wordpress site in less than 30 seconds.