OpenClassic is a custom Minecraft Classic server written in java. It has a basic plugin and event implementation along with an API at the moment, and many features.
OpenClassic Client is a modified Minecraft Classic client to have more features and to support the OpenClassicAPI.
OpenClassicAPI is the API for the custom server OpenClassic. See OpenClassic for details.
OpenNBT is a continuation of Graham Edgecombe's jNBT project. It has up-to-date support for the latest NBT specifications and a few custom NBT tags.
CanaryBridge is a compatibility plugin that allows hMod/Canary plugins to be run on the Spout server. It currently only runs a few plugins, but in the future it will run most plugins there are.
NPCCreatures is a Bukkit plugin allowing NPCs of any mob type to be spawned. On its own it doesn't do much, but other plugins can hook into it and use it in their features.