MCUpdater (or MCU for short) is a java-based update client for Minecraft. It is written with server administrators in mind and makes it easier for them to provide updates to their players when mods or configs change. It runs on Windows, Linux, and OSX.
Not really. MultiMC is a very sophisticated update client that allows players to customize their mod layout. It works well for people who want to experiment with mods or who want to maintain many separate MC configurations. It is best for single player applications where mod versions and config synchronization are not essential.
I use MultiMC daily - and I use MCU daily, they're for different things.
MCU doesn't provide an interface for players to drag and drop mods around. It downloads an XML serverpack definition from a web server. This serverpack tells it what mods and config files to download, and where to install them.
No. Most of those modpack launchers download bundles of mods and configs wholesale from their own mirrors and just unzip and launch them. If a new version comes out, it's usually a whole fresh download. It's also something of a walled garden - it is an exclusive club who determines what files go where, and their formats aren't usually very open or flexible. So if you mostly like Pack A but want to add module B and remove module C and tweak 2 or 3 config settings here and there, good luck.
Anyone with a Minecraft server can maintain their own MCU serverpack. The initial setup may take a bit of doing, but once you have it set up, making small changes is very easy. Then, if you decide to push a small config change to your players, you can just edit the config file and update the serverpack so the next time they update, it downloads the change for them.
When my players voted to add a creature mod to our old MC 1.2.5 server, it took me maybe 15-20 minutes to set things up for them. Then 2 months later, when they voted to remove the mod, it took me longer to reboot the server than it did to publish the config change.
Better instructions are forthcoming (as soon as I can write them). Until then, know that the server admin needs to write and host the serverpack somewhere. Players need to download a recent MCUpdater.jar and tell it the URL to the serverpack. Then, they need to *update* their installation before they can launch the game. If all goes well, the mods will download and install without problems.
We tend to hang out in #MCUpdater on esper.net and are usually happy to try to help people figure this stuff out if we're awake. The most frequent problems, however are caused by one of these things:
<MD5/>to cache files between updates (which you should be doing), make sure you're generating fresh checksums of the files any time you update them - a stale MD5 may mean that people don't download a file when they should - or worse, that they grab some other random unrelated file and try to use that instead.