I've seen that application but I forgot to note its name down, and now I can't find it:
You to place e.g. your laptop next to your desktop PC's monitor, you start the application in question on both and the laptop's screen becomes a second monitor to your desktop PC.
It's similar in a manner to VNC, TeamViewer and Remote Desktop, but while they can act as a second monitor in 'Clone' mode, this one simulates 'expand/span' mode - the native desktop of one PC spans screens of both computers.
IIRC the application allowed to mix Windows and Linux PCs freely (use Windows PC as host to display extended Linux screen, and vice versa, get Linux machine to display a part of Windows desktop).