What do you define as a RTOS?
I would consider QNX one of the best RTOS out there... QNX was designed as a RTOS, unlike RTLinux which is a hack.
PS. Gigger Hertz, definately give QNX a try, but be warned, it does lack the driver support of other OSs.
Well I've tried QNX 6.2.1, the latest version, but apparently my system of two 266 MHz 1 MB Pentium Pro chips with a gig of RAM is not powerful enough. I am downloading the QNX 6.1 ISO as we speak and hope that the hardware requirements are not so stringent (re: ridiculous).
You'd be better off with v4.x, (runs well on a single P55C@266MHz w/256MB Ram).
As Honold and Frank said QNX is not a desktop OS, it's a workhouse rather than a play thing... I do suggest you give it a try as a comparison between a workhorse and a toy... and for general desktop usage the toy OS may suite you better...
PS. QNX is normally used in embedded environments running mission critical stuff, and QNX (the company) provide a x86 version to assist developers code for the embedded environments on the same OS as the target environment. For a developer QNX offers a fantastic working environment, but if you want to play games, use office, etc don't bother with it... (I used it for a stint on several boxes as a data logger, dumping serial port to a text file, which it performed the job very well, but later moved them to Linux due to better SMB network and NIC support).
Well I've done my checking and QNX 4.x seems to be almost a decade old and very, very expensive. QNX 6.x, by contrast, is updated every 12-18 months and comes at the right price (free), is downloadable and a snap to set up. QNX 6 has SMP support and above all else is very efficient – I am all about efficiency.
Also, the P55C was a Pentium chip, of the 585 core. My dual Pentium Pros are the first iteration of the 686 generation, just to clear that up, though I am sure that QNX 4 would love a sweet spot like my dual overclocked Pros to run on.