I’m really enjoying ChibiOS.  The hardware abstraction layer is beautiful and the OS is rich, fast, and compact.  I am using it for a wireless IP gateway application.  I’m doing some pretty low-level stuff (interrupt driven SPI communications with a transceiver) and I have not had to write any processor-specific code!  I’m still getting to know ChibiOS, but I already have an application running on development boards that is communicating via the wireless transceiver, pulling an IP address via DHCP, serving a web page and a command shell via TCP, and also a separate serial command shell.  It fits in 80K.  By the end of next week, I hope to have a first cut of fully operational application software and move on to the custom hardware design.