In my view this is what you would want:
A: An area where users of Mux/Mulab will be able to browse and download content. They most likely want to be able to read descriptions, listen to , read comments, reviews and see ratings.
B: An area where "advanced" users may upload content to either be shared publicly (releases, see above) or privately with other users (collaborations, alpha/beta testing). Here you would like some kind of messaging system, version tracking etc.
There are third part options for all these things but you'll still need a server for content storage and a server for the web application(s).
1. Content storage
This will always, in some way, cost money. At least in the long run. Sure, there are Dropbox, Google Drive and a whole bunch of others, but the limitations of the free personal accounts for these services are very limiting. They may be helpful for a few people collaborating but not for public releases of patches.
So either Mutools could dedicate server space for this or a one could us a cloud server solution. As I mentioned above Azure S3 is one of those.
2. Web application
For this there are a lot of third party options. Both free hosted content management systems (Wordpress etc) and open source solutions that need to be installed and run somewhere. Here I mentioned AppHarbor since they actually have a free tier. Any half decent web hotel would also let you install applications that may run these things. Still need a lot of tweaking to get it the way you want...
If you focus on scenario A I believe a blogging platform would get you pretty far. I guess Wordpress and others have add-ons for rating systems and you get all the commenting etc that you expect from the user side of things. You also normally get a good tagging system for browsing. I also think it's possible to have a members area for up-loaders of patches. If using a free out of the box solution quite a lot of manual administration is probably needed though.
And remember, you still need somewhere to host the files.
Scenario B would more be in need of custom code. Michael L mentioned Github but I don't see it being a good fit for this. It's meant for code collaboration of open source software. It could work if the mux-files were in a humanly readable text format since then you at least could make use of the change tracking but they're not
I believe for now that these forum are enough for simple collaborations although I could see a more dedicated solution make sense if the demand is high. There's always also the possibility for a few persons to share eg a Dropbox folder.
Hope I'm not coming across as blunt or dismissive, I just think you need to know what you actually want
Although I could see myself setting something like this up I surely don't have any free time to dedicate to it now...
