As its name suggests the picoTK Toolkit is focused on small systems - where the emphasis is on simple presentation of information rather than modern interactive windowing systems. Featurewise picoTK is not comparable to modern full blown GUI interfaces like Qt/embedded or nanoX. Filling the need for many small applications where the overall overhead using more feature rich GUIs is contraproductive. picoTK has its own C programming API using only a few functions. Basically picoTK is an "output only" toolkit featuring the following:
fbe) for Linux/X11 using shared memory IPC. This is used for the development of the toolkit and can be used for the development of your application. That is you can have a simulation version of your application that run under Linux - and look at its virtual screen. Than you can cross-compile the same application for the target system. The framebuffer supports arbitrary sizes (up to 640x480) and 1, 4 and 8 bpp color depth.
http://www.oarcorp.com) you can have a nice graphical output embedded system using solely GNU technology.