If we're talking about things we WISH existed.. (que the sci fi music).. I wish I coudl mount some kind of electromagnetic (or more like electromagicalnetic) inertial siphon. So when I'm tired, I could "hook" to a car and get half a percent or so of their linear momentum, corrected to match my wheel direction.
I also wish there was some kind of magical 0 friction no lost energy drivetrain with infinite gain ratios, and frictionless hubs as well.