An other quite interesting concept (also a little less "out there" and actually, successfully tested) is the NERVA engine, also linked at the bottom of the article[1].

It uses a nuclear reactor to heat up a single propellant (liquid hydrogen) which then rapidly expands as it goes out the nozzle at the end, resulting in quite high specific impulse and is more efficient than a chemical rocket.

If we want to go really far out in terms of crazy rocket designs, there is also the concept of the nuclear salt-water rocket[2][3].

This one uses enriched plutonium or uranium salts, dissolved in water, inside a neutron-absorbent fuel tank design. The water is pushed out through a pipe, where an uncontrolled chain reaction starts. If it goes fast enough, it explodes right inside the engine bell, pushing the rocket. The proposed design in the original paper is supposed to generate hundreds of gigawatts of thermal power. This is comparable to the Chernobyl reactor #4 when it exploded, but unlike Chernobyl (which only exploded once) or the pulsed Orion engine, it could run continuously. The stuff that flies out of the nozzle easily reaches escape velocity of the solar system.

[1] https://en.wikipedia.org/wiki/NERVA?useskin=vector

[2] https://en.wikipedia.org/wiki/Nuclear_salt-water_rocket?uses...

[3] Obligatory Scott Manley video: https://www.youtube.com/watch?v=cvZjhWE-3zM