Ornithopters are planes that fly by flapping their wings.
Herbert's ornithopters were powered by a living creature that acted
like a hydraulic pump to flap the wings. Auxillary jets were available
for take off and limited use in flight.
2300AD Player's Guide includes a description of a kind of
solar-powered strap-on ornithopter that uses servo's to add power to
the pilots own arm motions.
In general, ornithopters are only good if you can come up with
some creative way to power them like the above examples. Otherwise
they're limitations (slow speed, huge wingspan, etc.) make them far
inferior to more conventional kinds of aircraft.