The most obvious items that runners need are shoes and socks, but since runners can be very particular about what shoes and socks they wear, they don't make a great gift.
Here are a few ideas that any runner would love.
Kahtoola Micro Spikes (https://kahtoola.com/product/microspikes/). These are the Cadillac of traction devices for winter running. They slip on over whatever shoes you're wearing and dig into whatever slippery sloppiness you might find yourself running in.
Foam Roller (https://www.tptherapy.com/). Runners are in a constant battle against injury, and one of the best tools in a runner's arsenal is a foam roller.
Buff (http://www.buffwear.com/). This is one of those things that you can never have too many of. I use mine as a headband when the weather is warm, as a hat when the weather is cold, and as a balaclava when the weather is ridiculously cold. Some people like using them around the neck (to keep it warm, to keep the bugs off, to soak up sweat), and some people keep one wrapped around their wrist for use as a snot rag. Plus, they come in all sorts of cool desigs.
Honey Stingers (http://www.honeystinger.com/). Honey Stinger makes a lot of great fueling products- gels, chews, cookies, bars etc, and they taste realy good. They make great stocking stuffers.
Handheld Water Bottle (http://www.ultimatedirection.com/p-603-jurek-grip.aspx?category=handhelds). The runner in your life probably already has some sort of hydration device but you can never have too many hand held water bottles- they take a fair bit of abuse on the trail (for example if you use them to break a fall) and can get a bit gunky over time so an extra water bottle is always good.
Re-Usable Water Bottle (http://www.bamboobottleco.com/). It's nice to have some fresh water waiting for you after your run- why not keep it in a cool, environmentally responsible bamboo water bottle?
Gift Cards- when all else fails, you can't go wrong with a gift card to someplace like http://runningwarehouse.com. They have a huge selection, and good prices.
So there's a few quick thoughts on Christmas gifts for runners. I'll be re-visitting this as we get closer to the big day. So long for now!