Do you own or use a one page app?

For those who don't know, a one page app focuses on one single task only. It usually does not have any option to create an account and if it does have an option then it is still not mandatory. They are generally free to use but you enjoy advanced features with their paid account. For example I have saved ton of bandwidth by compressing images on https://tinypng.com/. I was trying to find more such apps and ended up with 10-15 apps which seems awfully less than what I expected. Since they are hard to find right now so I am building a list of every useful one page app out there ( useful being the keyword ). Please share the URL of one pages apps that you use or you own. Shameless plugs are welcomed.