I have been almost single-handedly maintaining and expanding one of the largest open source PlayStation 1 SDKs [1] over the last year, adding all sorts of things from video playback to a dynamic linker. There are still plenty of bugs to fix and things to improve, but I think it can already be considered production ready for the most part.
Unfortunately, however, the tooling around PS1 development still has a long way to go; while there are e.g. several open source image or audio conversion tools, none of them are on par with old Sony tools in terms of features. In fact most people still use the official SDK complete with its 1998-era compiler, despite its bloat and heavy use of undocumented proprietary file formats.
Though I believe Sony made the graphics chip largely unavailable for use on the Net Yaroze (at least directly) as a opposed to a true dev kit where developers had access to the full hardware.
