Remmina - Remmina is a remote desktop client written in GTK+, aiming to be useful for system administrators and travellers, who need to work with lots of remote computers in front of either large monitors or tiny netbooks.
TeamViewer - TeamViewer lets you establish a connection to any PC or server within just a few seconds.
rdesktop - rdesktop is a free and open source remote desktop protocol client designed to make the users able to manage their PCs in a new way.
mRemoteNG - mRemoteNG is a fork of mRemote, an open source, tabbed, multi-protocol, remote connections manager.
KRDC - KRDC is the KDE Remote Desktop Client.
join.me - Instant screen sharing. Instant Aha!