The current front runner in this niche is Sponge.

"Sponge is a combination of a new API (based off of Spout/Flow’s APIs) implemented on top of Forge, with assistance from other parts of Minecraft’s modding community (Glowstone, Cauldron, Spout, etc.).

It will be both a server and client API, and its target user base is pretty much anyone that wants to mod their game, including server owners. However, we may focus on the server-side portion first.

We invite any developer to help out."

https://www.spongepowered.org/

Development seems quite active on Github.