VLC
This page describe things what you can or cannot do with VLC.
VLC is easy to install on Windows (r), OS X or GNU/Linux. run the same way and do same things on all those platform.
Another interesting thing in VLC is the plugins architecture and the AJAX web interface.
Plugins can be used for example for digital zoom, transcoding, mosaic with 4,6,9,16,... cameras, realtime processing, ...
I have successfully recorded & transcoded to mpeg4 video and viewed the result in the same time. It work on my notebook. (thinkpad T60 Intel Core2 2GHz, 2 GB RAM)
so the first basic thing you can do with VLC is to play both unicast or multicast stream from the camera.
just lunch:
vlc rtsp://192.168.0.9:554
Replace 192.168.0.9 with the IP address of your camera.
VLC is available under Ubuntu GNU/Linux and most of the other GNU/Linux distributions. For other OS visit VLC homepage here http://www.videolan.org/vlc/