[NOTE: I have since made a better version here.]
This ISO image of a bootable CD is the combination of two different threads I have been working on: how to make bootable CDs and how to set up RT-11 with Basic in SIMH, the PDP-11 emulator. This CD only works if you have an IDE CD-DROM drive though.
So I took the bootable CD image that I had already made and added SIMH for MS-DOS. Then I added my RT-11 / Basic-11 disk image. I set SIMH to start up automatically from a 16Mb RAM disk. It all seems to work. It is pretty fast in comparison to a real PDP-11! Because it is running from a DOS RAM disk, you get read/write access under RT-11 (until you switch the power off). But it does mean that you can't mess it up since you get a clean install each time you boot the CD. Oh, and if you're looking for a manual on Basic-11 I've posted some stuff here.
So... if you'd like to have a play with an ancient operating system, give it a try. Burn the ISO to a CD or boot it directly in Virtual PC or VirtualBox. I've tried it with as little as 32Mb RAM, and it works fine. When I get time, perhaps I'll do an ancient Unix Live CD as well.