The app building sites he refers to are:
If you're interested in building apps, also have a look at MIT's free App Inventor (which will seem very familiar to you if you've used MIT's Scratch programming software.
Many teachers are writing apps for their classes. One is Jason Welker - you'll find his Economics apps here.
Do you have to be a teacher to make apps?
"A 10-year-old has invented an app that keeps track of reading homework...Students are required to turn in a reading calendar every month. Daniel designed an app where you click the day and enter the number of reading minutes and then save. At the end of the month, hit the email button and the homework is sent to the teacher." Read the news story here, and watch the news clip below.
Besides that it's fun, and an intellectual challenge, are their other reasons why teachers and students should investigate app building?
ISTE calls it "Computational Thinking"