I am looking for a very low speed motor (under 30rpm) to move small loads. What do you suggest?
The question of “how slow can a motor go?” is entirely up to motor manufacturers. The most efficient way to reduce speed is with a planetary geardown. A worm gear reduces speed drastically in one step, but most are inefficient. If you are looking for under 10 rpm, the geardown will be in the order of 1:1000. Lowering the supply voltage to a motor (within the design limitations) also reduces the speed (this can also be done with PWM).
Small motors:
RB-Tam-03, 1.5V, 13.5rpm
RB-Tam-04, 3V, 25.5rpm (extra planetary gears reduce even more)
RB-Tam-14, 1.5V, 4rpm
RB-Tam-15, 1.5V, 1.3rpm
RB-Tam-30, 1.5V, 20rpm
RB-Owi-30, 3V, 8.7rpm
Medium size motors:
RB-Ban-10, 7.2VDC, 63rpm
Many stepper motors can be run at low speeds, though there is added complexity in control.