This is a very open-ended question; depending on what you want to do with such an arm would be important to know what to recommend. Do you need fine control and

This is a very open-ended question; depending on what you want to do with such an arm would be important to know what to recommend. Do you need fine control and accuracy? Do you need speed, or strength, or some combination? What kind and size of work envelope? Cartesian vs revolute vs...? Have you considered going "old-school"? That is, picking up a used Microbotics Teachmover can be easily done - though you'll probably have to hack up a controller for it. Similarly, there are old Rhino Robotics arms that can be bought fairly cheaply (though I warn you - they are very heavy). There are several other robot arms from the 1980s that you can find if you look, but all of them will require more than a bit of "hacking" to get working with modern computer systems. Have you considered building an arm using Lego or some other similar building kit (Meccano, Fischertechnik, KNex)? Or maybe parts from Servo City, or 80/20 extrusions? What about hacking a conversion? Like converting an old Armatron to be computer controlled (not easy to do, but it is possible - it was done more than once back in the 1980s, and those articles on how to do it are still able to be found). Note that it is easier to convert the Radio Shack Armatron that has a wheeled base, than it is to convert the joystick controlled Armatron (the latter requires a bit of mechanical skill and knowledge, on top of the electronics to interface it). Another possibility (though fraught with even greater issues) would be to find a second-hand laboratory industrial robot arm, and get that interfaced. There are more than a few out there which were electric powered (mostly - some have pneumatic end-effector attachments), and are small enough to play with in a "home" setting. The downsides are lack of inexpensive replacement components if you need them, among other issues.