Ehhh, 'applications of the venerable 555'. The internet is full of pages, some completely dedicated to this little chip - and rightly so!
Aside from the usual time delay monostable, or square wave astable, and PWM, I've used these little bad boys for a number of things. They're cheap, readily available (literally everywhere) and have some basic building blocks that can either be used on their own, or together. For example, the output stage can drive a nice ammount of current, I've used it for MOSFET driving, not external components, really just a high current buffer. The two opamps inside form a convenient window comparator. It can be used as an RS latch, and can be the basis for very cheap (and not always efficient) switch mode power supplies. step down, step up, even sepic tologies can be done with a stock 555, albeit with frequency limits.
.... low battery circuits, power supply sequencing, charge pump converters, panel indicators, adjustable clocks for low-speed logic.. the list really does go on.
There are often much better ways o doing things, but for cheap and simple 'sub circuits' where there are few limitations it has myvote.