Yes, you are free you use custom server JARs, including, but not limited to:
- CraftBukkit (including unstable beta and development versions)
- Spigot
- Tekkit
- FeedTheBeast
- BungeeCord (as both a hub, or servers in a BungeeCord network)
Yes, you are free you use custom server JARs, including, but not limited to: