At first glance, this deskside coffee accessory seems silly, if not teetering on the edge of unnecessary. This small black plastic disk with a cord is the Mr. Coffee Mug Warmer for Home/Office Use, and for just $9.99, it promises to keep your coffee hot for hours at a time. But here's the thing about the Mr. Coffee Mug Warmer. People love it. After nearly 4,000 reviews, the Mr. Coffee Mug Warmer has a 4.1-star rating out of 5, and it's one of Amazon's best-selling coffee accessories. It's the kind of coffee accessory you never knew you needed until you had it, and then, once it's on your desk, you wonder how you ever drank your office coffee without it.

The concept is simple. Take the cord that comes with the device, plug it into a regular electric socket, and the center of the mug warmer will heat up. Place your mug of coffee, or tea, in the middle, and you'll have hot coffee for hours, no microwaving required. The best part is that it works, according to dozens of satisfied Amazon customers. It keeps beverages "drinkably hot (not just lukewarm like some other mug warmers I've tried)" writes one customer. "I use it all the time when I'm home as I'll get a cup of coffee and just forget it on my desk," writes another, with an accompanying five-star rating. "Really saves me from having to go back to the kitchen to warm it back up." It also works to keep pots of tea hot, as one reviewer pointed out

The Mr. Coffee Mug Warmer is not, however, a miracle worker and cannot reheat a stone cold cup of coffee. Plus, the need for a traditional electrical outlet can be a bummer if you're low on desk space. 

But it's worth the hassle. The USB-powered mug warmers are generally not as effective, generally only garnering two- or three-star reviews on Amazon, and the Mr. Coffee Mug Warmer is definitely going to be the best bang for your buck. One Amazon customer said that the heated pad got so hot that drops of water sizzled off of it. That also means it's kind of a fire hazard; another customer, for example, left their mug warmer plugged in overnight, reportedly to heat their coffee, and it literally burned a hole into their desk.

So don't leave this plugged in at the office after-hours, but if you're looking for a way to keep that fresh cup of joe fresh for as long as possible, invest the $10 in this little guy and save that extra trip to the office kitchen.