Adventure Lobbies was a free server hosting service for Minecraft that let gamers invite their friends to play popular user-created content with them. Over the course of its existence, the service provided a selection of over 200 user-created content (known as worlds/maps/adventure maps) and offered the ability to generate random worlds. Sponsored by large YouTube channels and powered by donations from the community, Adventure Lobbies provided over 1,000,000 free gaming sessions to the Minecraft community.
Here is the Bukkit plugin which powered it, developed while I was in high school.
Not included, at this time, is the front-end, user-facing website that worked in unison with the Minecraft server-side code.