RTOS

• http://sources.redhat.com/ecos/
• http://www.on-time.com
• Realtime Mantra: Embedded and Realtime System Design
  
• http://www.terekhov.de/DESIGN-futex-CV.cpp
• http://www.jaluna.com/
• http://www.oreilly.com/catalog/posix4/
• http://people.mech.kuleuven.ac.be/~bruyninc/rthowto/
• RTEMS
• ChorusOS
  

RT FAQ

http://www.realtime-info.be/encyc/techno/publi/faq/rtfaq.htm
http://www.groupipc.com/rtfaq.htm
http://www.faqs.org/faqs/realtime-computing/faq/
http://www.cis.ohio-state.edu/hypertext/faq/usenet/realtime-computing/top.ht

> I want to get some experience with RTOS other than Real-Time Linux and
> AMX-86/MS-DOS which I've used until now.  I have an old '486 taking up space
> which I'd like to use as a test-bed for this project.  Are there any
> available OS that I can use for this purpose?

Many.  Off the top of my head: RTEMS, Jaluna, Roadrunner, MenuetOS.
If you just want a learning project, you could even try QNX.