Synergy - Cross-platform software for sharing your mouse and keyboard between multiple computers
Input Director - Control multiple windows systems with one keyboard/mouse. Share a keyboard and mouse across multiple windows system.
Barrier - Barrier is a cross-platform software for sharing your mouse and keyboard between multiple computers...
Ultramon - UltraMon is a piece of software built to help with the management of multiple screens on the same computer system. Without software like this, taking full advantage of an expanded desktop space can be difficult.
Multiplicity - Multiplicity enables a user to control multiple computers with one keyboard and mouse.
DisplayFusion - DisplayFusion will make your multi-monitor life much easier.