A today added support for Xbox 360 and Xbox One controllers, as well as any generic X-Input controller. What this means is you can now use practically any gamepad you want through Steam as if it were a Steam controller—without being forced to actually use a Steam controller.
This is a continued expansion of Steam's controller support after they last month. It makes sense that Xbox controllers would be hot on its heels, and the inclusion of X-Input support means that Valve now has most gamepads out there covered. It also means that you likely won't ever need to fiddle around in JoyToKey or a similar keyboard-to-X-input mapping program ever again, as you can just map it all through Steam or download someone else's profile who has already done so.
Another nice perk of this support is that you can save profiles to specific controllers that Steam will remember each time you plug them in, as well as use the Steam controllers rather robust set of tools for mapping complex button presses or mouse movements. While I'm not crazy about the Steam controller itself, opening up its customizable toolset to more comfortable gamepad alternatives is actually pretty exciting.
You can opt into the Steam beta client by going into the "Account" tab of your Steam settings, selecting "Change…" under the "Beta participation" section, and choosing "Steam Beta Update" from the dropdown menu. To make non-Steam controllers customizable, you'll need to enter Big Picture Mode, click the "Controller Settings" tab in the settings menu, then check the boxes to enable configuration support for whatever controller you are using. You can then choose a game, click "Manage Game," and then "Controller Configuration" to customize it to your heart's content.
No word on when this update will move from beta to live, but it took the PS4 controller support a little over a month.