There are too many commercial embedded operating systems to list here, but if Linux isn't to your liking, here are some other open source operating systems which share some of the same benefits, and have been ported to embedded PowerPC processors:
http://sources.redhat.com/ecos/
eCos is an open-source, royalty-free, highly configurable, application-specific operating system ideal for embedded systems development. It is targeted at high-volume applications in consumer electronics, telecommunications, automotive, and other deeply embedded applications.
RTEMS is a real-time executive which provides a high performance environment for embedded applications including a POSIX 1003.1b API, TCP/IP stack, multitasking, debugging and filesystem support.
http://people.FreeBSD.org/~picobsd/
For those of the BSD persuasion.