Unlike other open source SDK projects for the PS1 that have come and gone, the main goal of the PSn00bSDK project is to develop a 100% open source SDK for the PS1 that tries to be on-par to the official SDK in terms of hardware support as possible. While PSn00bSDK is still a work in progress, earlier iterations of it it was fit enough to create n00bdemo as it already has the best graphics and 3D support compared to other SDK projects that I'm aware of.
Other components such as SPU, MDEC, and better CD-ROM support have yet to be implemented so proper homebrew games that utilize the hardware properly can be made with this SDK. But after several updates, it should be fit enough to make some interesting things with this SDK now that controllers and basic CD-ROM support have since been implemented.
Another neat thing about PSn00bSDK is that it follows the same library syntax as the official SDK as closely as possible not only for familiarity reasons but also to make it easier to port existing homebrew over to PSn00bSDK with little to no modifications, provided the functions it depends on have been implemented.
Screenshot of n00bdemo taken from pSX/psxfin:
Video on PSn00bSDK focusing on the early development process of this project:
Webpage (has a precompiled Win32 build of the GNU GCC 7.4.0 compiler and internal SVN repository):
Github repo (has more info and build instructions):