Jitsi is a great collaboration tool e.g. for doing maintainance and providing support at the far end. However, since some versions desktop sharing seems to be missing.
I cannot find an option on my side to enable it. I can see the other screen but cannot do anything there. Even with an old version of Jitsi which still has that option I wasn't able to work remotely from my Jitsi 1.1.4368.10168 on Ubuntu 12.04.