Which free graphical libraries exist for NXP's processors?

There is emWin from Segger that is free for NXP's processors. See here for details about usage and licensing conditions.

There is also the SWIM graphical library which NXP describe like this:
"Simple Window Interface Manager (SWIM) is a basic graphics library developed for the NXP LPC products. It can be also be used with LPC controllers that do not have a dedicated LCD interface. The SWIM graphics library allows developers to quickly and easily implement a system with basic graphics support. SWIM support basic graphics primitives such as lines and squares. Simple non-overlapping windows with logical coordinates per window are supported."
Check out the details here.