Out of Wikipedia's long list, the names I recognized as "deployed" amongst people I know:
eCos, LynxOS, QNX, RTAI, RTLinux, Symbian OS, VxWorks, Windows CE, MontaVista Linux.
Of these, I'd guess this order: Symbian OS, Windows CE, QNX, MontaVista, RTLinux. But those are my guesses. :)
If you want to include Soft Real-Time systems, Windows 7 and stock Linux might qualify. Depends upon your needs.
