Gather 'round for story time, kiddos. I did this a loooong time ago now:

Back before Minecraft hit version 1.0, I was an administrator for a public server and was inspired by some of the early economy plugins some people wrote. Well I ended up writing a plugin that provided a global shop that the player could interact with via the chat interface. The plugin was backed by yaml configuration files and was capable of using multiple economy plugins and permissions. All in all, it was a neat little plugin and it started to gain a little popularity before I ultimately passed off the project to another developer. I decided to stop maintaining it because around that time, the game and the mod framework versions were not always in sync and changed quickly. Every version change would break some aspect of the plugin and I would've needed to maintain multiple versions to accommodate all the people using it.

It was a worthwhile project for me because I learned Java and got a better feel for object oriented programming.

If you're interested, you can look at the old source:

P.S.: Sorry for the crappy screenshot. It was the best I could find.