The Incredible 555 : Here are some 555 circuits, The most versatile and simple chip of all time ! - Signetics made the 555, This is one IC that is like a E-LEGO, this along with Op-Amps and other components can make many interesting projects.

Some of my 555 Circuits are at my blog - 555-Circuits - delabs Schematics

On some pins like Control Voltage it responds to analog signals and on Reset and Output the signals are Digital like. The 7555 a CMOS version consumes lower power and oscillates at higher frequencies. 

The best part of the 555 is that it has an voltage detection. It has a Versatile Logic which is defined by voltage comparators. The ability to loop it like an opamp for feedback makes it the first "Programmable" Mixed Chip. In short it make an ideal building block.

555 Timer Circuits at delabs

555 Timer based Circuits