Synergy (Software) Lifetime Basic Plan for Free
From their website:
Synergy combines your desktop devices together in to one cohesive experience. It’s software for sharing your mouse and keyboard between multiple computers on your desk. It works on Windows, Mac OS X and Linux.
A Basic plan (normally $10) gives you all features except for SSL encryption and support.
If you want, you can upgrade to a Pro plan for lifetime for $19 (normally $29) after redeeming the coupon.