I've been using this t-shirt folding technique for years and I'm surprised at the amount of people who don't know about it. With this method, you can fold a t-shirt in mere seconds.
My partner and I both own a ton of t-shirts and if I didn't know this folding trick I'd have gone mad from spending too much time doing laundry. I learnt this technique from a YouTube video years ago and when I demonstrated it to a few friends recently, they were amazed at how quick it was to fold a t-shirt.
No special tools required; you just need your hands. It's not only about folding clothes quickly, the t-shirts also end up being a consistent size so that you can stack them neatly. This technique also works on singlets.
The original video is over 10 years old and doesn't contain any helpful pointers so I've included a slightly newer clip with detailed instructions. Enjoy!