Best way to remote login on a Ubuntu machine from Ubuntu/Windows and OS X

Solution 1:

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. Remmina supports multiple network protocols in an integrated and consistant user interface. Currently RDP, VNC, NX, XDMCP and SSH are supported.

Remmina is released in separated source packages:

 "remmina", the main GTK+ application
 "remmina-plugins", a set of plugins
 "remmina-gnome", the GNOME desktop applet
 "remmina-xfce", the Xfce panel plugin

Remmina is free and open-source software, released under GNU GPL license.

screenshots : here

Solution 2:

In the old days you started xdm on the machine, and it provided remote X logins to X servers started with "-broadcast" or "-remote" (if I recall correctly).

I believe I have seen that Ubuntu can enable this X display manager, so I would recommend you have a look at if it is still possible.

Solution 3:

Cendio Thinlinc can be used with 10 concurrent users in production systems for free http://www.cendio.com/products/thinlinc/. I haven't administered it, but I like the client.