Freeware version:
http://help.netop.com/download/current_rc/DOS/updateuk.htm

On the host run "REMPC x /k:3"

where x=computer name.

On client run "NETOP"

Once the session is running hit CTRL+Z for fullscreen.
Hit ALT+E to bring up the menu.

Speed is pretty fast. It looks like it only works for text mode but according to the documentation you can get it to use other video modes and 16 color graphics.

Looks like using the switch "/K:3" on the host allows the remote pc the ability to enter commands on the host which means if people wanted to play an text game like zork together they could!