I am running Ubuntu 8.04. How can I configure my environment so that another user can log in to my machine and see my desktop and X programs (e.g. firefox) (i.e. ssh won't work for me).
For simple screen sharing, use vnc. (RealVNC download)
Otherwise (assuming Windows OS), for actual logging into a new session, you would configure a new account with remote desktop privileges and enable the Remote Desktop service (previously known as MS terminal services)
On the server:
tick 'Allow users to view/control your desktop', choose password etc
on the client to connect. If on win you can use normal vncviewer, since remote desktop uses a VNC on Ubuntu.