The RSPS scene was really big, active, and very competitive. What was so fun about this is that the private server communities always encouraged users to share data resources, programmijg tutorials/snippets, and reverse engineering documentation of the game so that way it helps everyone that's trying to program their own server. Everyone wanted that "Java Developer" icon next to their forum name, and to do that was to contribute back. There's only one community standing left and it's an absolute gold mine of programming & game-development information [1]
It's a very niche community, but back then anyone could spin up a server and quickly have 50+ players actively playing within a week, and because of that, i've learned the concepts of client to server network designs, mysql databases, asynchronous communications, non or threaded model designs, and the whole Java basics spectrum.
Another good venture is the MineCraft server development communities such as Bukkit,Technic, Sponge, and Spigot-MC [2]. They've got some interesting frameworks as well for the Mobile-Portable edition that's pretty interesting to look at.
Last I can think of is Garry's Mod. GMod solely revolves around Lua. [3] Roblox is Lua as well and they pay a good chunk of money to developers if you can create a trending server.
But you've got to have some sort of real interest to get into these, otherwise it feels like a waste of time imo. I was young and havent graduated yet so I had all the time to mess with these source-codes & actually host them, but now, I just don't see the point unless i'm going to attempt making money from it. It's a rewarding and fun experience though.
I wish I knew more underground communities espcially akin to RSPS, it was the best learning experience and it is not often you find something genuine like that.
1. rune-server.org
2. spigot-mc.org bukkit.org
3. gmod.facepunch.com roblox.com