In principle, almost anything: any system of masses will emit gravitational waves with an intensity proportional to its mass quadrupole moment (which may be 0,

In principle, almost anything: any system of masses will emit gravitational waves with an intensity proportional to its mass quadrupole moment (which may be 0, as it is for rotationally symmetric systems). But the proportionality constant is extremely small. Realistically you're looking at stellar-mass objects, if not larger.