Linux should work fine, and is tested all the time. Serial support needs nothing special (only the serial driver for your serial device), but usb support on linux has a few requirements:
usbfs /proc/bus/usb usbfs defaults 0 0
(replace "usbfs" with "usbdevfs" for linux kernel 2.4.* - will work on kernel 2.6.* too)
OpenCT should work, but this wasn't tested for sometime. Latest OpenCT seems to not find libusb, we are working on it.
Latest OpenCT supports Solaris fine and was tested to work.
Sunray including client/server architecture was recently added to OpenCT and 0.6.5 once release should work fine. Till then please use a snapshot or svn trunk checkout.