The 555 timer integrated circuit was invented in 1971. Over a billion are made every year, because they are so versatile. Charles Platt wrote of the chip:
It has turned out to be the most successful chip in history, both in the number of units sold (tens of billions, and still counting) and the longevity of its design (fundamentally unchanged for almost forty years). Even now, about a billion 555s are manufactured each year.
In Make: Electronics, I decided to include the 555, because it remains so fundamental. It's also a wonderful teaching tool, since it can be used in so many ways. If you want to build, say, a reaction timer, using a counter and a couple of logic chips, you're going to run it with a 555 timer, and you may end up adding a couple more 555s to take care of functions such as delaying the start of the count and locking the display until a reset button is pressed. You can also run a 555 fast enough to generate audible tones, which can be incorporated into a burglar alarm, or you can use it in a combination lock. All three of these projects are included in the book.
Make magazine celebrated the 555 timer and its creator, Hans Camenzind, a few years ago when it featuring a 555 Week on its website.
555s are dirt cheap, too. Ebay sells 100 for $(removed).
Because the 555 is so fundamental electronics, people honor it by making giant size versions of it. In the photo above, you can see the 555 storage ottoman Ross built.
Here's a similar project — a 555 timer stool:
I also like the 555 footstool that Windell and Lenore of Evil Mad Science built: