Crossloop

Crossloop is a great remote access and assistance tool which is very easy to use and works behind NAT firewalls. Perfect for situations where you need to demonstrate some thing or other.

Basically, one computer is the host who will be sharing their screen and the guest who will be able to see the hosts screen and take control of the keyboard and mouse. The host still has access to their screen and can see everything the guest is doing and can terminate the session at any time.

Crossloop

Synergy

This is a neat little sourceforge project that allows you to use a single set of keyboard and mouse with multiple computers with monitors at a single workstation. Its cross platform and allows simple configuration where you can arrange screens, and once working, provides seamless usage of multiple computers.

Synergy