Steam has had native support for PlayStation 4 controllers since last month, and with today's update that has extended to Xbox 360 and Xbox One controllers, as well as any controller using the generic X-Input. In other words, no matter what sort of a controller you use, it should now work on Steam without any extra effort.
This might not sound like big news, especially since most games already support Xbox 360/One controllers, but if you ever had to mess around with JoyToKey or other such key mapping programs you know exactly how welcome this update really is. Nothing is better than just plugging in your controller and simply pressing start to play, without the whole 15 minute procedure that sometimes goes before it!
There are some limitations to this however, as noted in the update itself: "Note that because X-Input currently lacks per-controller means of unique identification, all controllers of that type will share personalization and configuration settings. As they share the same inputs, Xbox 360/One/Generic controllers will all see each-others configurations when browsing. Automatic conversion will be attempted when loading configurations from other controller types."
Besides adding support for Xbox controllers, this update has also extended support for various third party PS4 controllers. In other words, Steam now natively supports some HORI, MadCatz, and Armor pads and fighting sticks. There is also a whole host of other, smaller changes in the update as well, so if you're interested in the full details you should head over to the official patch notes.