Islands in the sky adds a new world generation type (imaginatively named: IslandsInTheSky). This is a void world, appropriate for playing skyblock worlds. Spawn starts as a 5×5 grass square, on top of a bed of hardened clay. Each island is 320×320 blocks, with a 32 block buffer space between islands, and these values can be changed in the config file.
To set Islands In The Sky to be the world generator used by a server, you need to edit the server.properties file, and set “level-type=IslandsInTheSky”, after which, you will probably need to delete the world folder to regenerate the world as a void world.
Islands In The Sky adds four commands. /island (or /is), /spawn, /invite and /accept. /Island will take you to (and create, if necessary) your own private island, and /spawn will return to you spawn for easy interaction with your servermates. /spawn is not created if another forge mod already adds it. (Not sure how this will interact with the various bukkit mods.) /invite and /accept are used to share islands between players. Hopefully the uses are obvious.
Islands In The Sky will also give you a starting inventory when you create your island, currently it is 64 bonemeal, and 4 oak saplings, this is configurable in the config file.
Islands In The Sky was created as a companion mod for Parachronology, to allow for multiplayer servers. You are free to include this in the server side of any modpack you might like. It is NOT intended for client side use. It shouldn’t break anything, but I won’t support client side use either.