If you’re looking to make a robot or other electromechanical device, there are a wide variety of options for rotational motion. Servos, steppers, and DC motors are available configured in many different ways, at all price points.
If, however, your device needs linear motion, options available at a hobbyist-level become more scarce. You could use a solenoid, but they don’t give you much control. Alternatively, one could hook up a servo motor with a linkage, or even put together a lead screw setup, but this adds mechanical complication that you may not want to deal with.
For another option that won’t break the bank, seller Morai Motion offers linear actuators from a 1 inch stroke to 6 inches. Available actuators traverse at a speed of .51mm per second and are able to push with a force of 4.4lbs (or 2 Kg-force if you’d like to be consistent with your units). So while not very fast, they can be precisely controlled and won’t shoot whatever you’re working on into the air! Check out the 1 inch stroke model in the video below, or see the Morai Motion store for more options.
If you need a linear actuator with just the parts you have lying around your shop, you can always made one with a glue stick and hobby servo. Although it perhaps won’t be as reliable as the actuators seen here, it is able to generate several Kilograms of force!
