NuttX is yet another small RTOS for microcontrollers. Works best on not-so-small ones (128kB FLASH, 16kB RAM). LPC2148 suits this very well.
What attracted me to it was the list of ports, one example program (nsh which is a small shell, which is great for interactively testing), and the existence of a graphics library and the great support from Greg Nutt, the main developer.
Time from reporting a bug to fixing it: less then 2h. Take that, Sun and Veritas!