Despite being open source, Synergy charges for builds of their binaries. Cost of $10 for Synergy Basic, or $29 for Synergy Pro. Older versions of the binaries are available for free, and the source code can be downloaded and compiled manually.
Only one computer can act as a host at a time, and the mouse/keyboard sharing is one way. So if you have computer at home, and a computer at work and you want to link your laptop to different computers depending on where you are, you need to change the host settings for each location.
Multiplicity enables a single keyboard and mouse to control up to nine PCs each with a dedicated display monitor. The cursor can be smoothly moved directly to any monitor regardless of which PC the monitor is connected to.