The Nano-X Window System is an Open Source project aimed at bringing the features of modern graphical windowing environments to smaller devices and platforms. Nano-X allows applications to be built and tested on the Linux desktop, as well as cross-compiled for the target device. Yale Huang ported Nano-X to the PSP, it is still an unstable version and might not be useable for everyone.

http://yaleh.blogspot.com/2005/07/nano-x-for-psp.html