Free software friendly Gamepads

Most are fine. What you mainly need to watch out for is compatibility with Linux. The Linux Joystick Driver works with XInput (the non-standard API used by Xbox 360 controllers), and any USB HID class controllers will of course work (it's a USB standard).

The only source of incompatibility I have come across (though there may be more obscure ones I don't know about) is DirectInput, a non-standard API used by DirectX. Controllers that send DirectInput data can't be recognized by the Linux Joystick Driver. The biggest offender I have come across when it comes to using DirectInput is Logitech, though I think most of their recent controllers have an option for XInput. However, I'm sure there are other controllers that send DirectInput data.

If a controller is said to work with Linux, or advertises being a standard USB HID, you can get it without worries (other than the quality of the controller, of course).

If Linux is not mentioned, you might be able to find other clues in user reviews. For example, USB HID is a standard supported by all operating systems, so if someone mentions not being able to use the controller on a Mac, you can be pretty sure it won't work on GNU/Linux either. If you are unable to pinpoint for sure whether something is compatible with Linux, make sure you buy from someone who will accept a return for a refund if the controller is of no use to you.

I recommend avoiding Logitech controllers; Logitech has absurdly terrible customer support, and both of the Logitech controllers I've had in the past broke in a weird way after a couple years (they stopped holding a connection).

Other than that, I'll comment on some controllers I've used (including some comments about quality):

- Gravis Gamepad Pro (USB version): Standard USB HID. D-pad is a little mushy, but pretty decent, and the buttons are perfect. You would have to get it used (it hasn't been manufactured in over 10 years), but they seem to have held up well.

- PS3 Sixaxis or DualShock 3: Standard USB HID, with the minor problem that the button and axis configurations are absurd. The tilt sensors work, though this is actually kind of annoying when configuring controls. Rumble feature on the DualShock 3 is non-standard, so it doesn't work. Most buttons are also axes, which is a little annoying, but usually doesn't seem to cause problems. D-pad is excellent, analog sticks are decent, and buttons are perfect, but the trigger buttons are pretty crappy.

- Snakebyte Final Fantasy XIV controller: Standard USB HID. Rumble feature is non-standard, so it doesn't work. As far as I know, the "Function" button also doesn't work. D-pad is not very good, but analog sticks and buttons are perfect.

- Logitech controllers: I've had two of these; one used only DirectInput, and therefore didn't work with Linux, and one used either DirectInput or XInput (controlled by a switch). Both of them were in general good controllers, but they both stopped consistently holding a connection after a couple years. I recommend staying away from Logitech controllers because of this experience.

- SNES controllers: The Mayflash SNES Controller Adapter is a standard USB HID, and works perfectly. The Gtron adapter is also a standard USB HID, but seems to be much lower quality, so I don't recommend it. I can't vouch for any other similar adapters, since I have only used these two. One thing to note, some SNES controllers made by Nintendo just don't seem to work with these adapters, for some reason. This seems to be the fault of the controllers, not the adapters.

Thank you very much onpon