The short answer is - many! RTEMS supports many different architectures. A few are listed here, but see this page for more detailed information.
- Altera NIOS II
- ADI Blackfin
- ARM
- Intel/AMD x86
- Lattice Mico32
- MIPS
- Motorola MC68xxx
- Motorola Coldfire
- Moxie
- PowerPC
- Renesas H8/300
- Renesas SH
- SPARC