Shift4Me is a system that you can mount on your bicycle. It shifts your gears, according to the pedalling cadence. If you set it to a desired cadence of (for example) 60RPM it will downshift if you're pedalling faster, and upshift if you're pedalling slower than 60RPM (with a tolerance). It is designed as a hassle-free, lightweight gadget that has one purpose: to make cycling more comfortable.

A lot of people don't shift their gears enough to maintain an ideal cadence,
resulting in an uncomfortable and non-efficient ride.
Shift4Me shifts the gears for you, so you can enjoy a comfortable ride without having to worry about being in the best gear.

Shift4Me is not a ready-to-use, commercial item.
It is an Open Source project that you can build and install yourself.