YourSiteAdmin is a custom-built CMS system by Alex Hall for the purposes of combining separate admin system for some websites built for different clients. Because these admin areas consisted of pretty much the same thing an idea was born to create a system that could cater for all current and, hopefully, future sites as required.
Since it's beginnings in 2012 the admin system has come a long way and grown vastly to cater for new ideas/requirements/types of site from new and old clients. The codebase has also undergone vast speed and efficiency changes to help improve things for both the clients and the developer(s). And there is plenty more still in the pipeline!
One of the major development processes ongoing at the moment is making the whole admin system responsive (so it looks good no matter what resolution or size your browser is, even on mobile). Some areas are done and work quite well no matter what size screen you are using but because there is so much to create some areas are still lacking in these areas.
We are also currently working on a few new solutions. The CMS was originally intended to just be an area for personal clients to edit their site content, but as interest has grown there have been mumblings of a requirement for an API to serve the content that is created in YourSiteAdmin so it can be used by other developers. The API is up and running but is so far very much incomplete. We do have one site currently running entirely off it though, which you can see at Morecambe Sunsets. The Api documentation can be found here, but is again fairly incomplete.
And once the API has been finished (to a point that it can be used to create all aspcts of a site anyway) we are looking to release a complete sitebuilder that allows you to create a site, in full, all from within YourSiteAdmin! The roadmap for this has not yet been completed but some development time has been spent starting this section of the CMS. We'll let you know when we're there!