The classic 555 timer on a breadboard:
Here, I will show how you can make your own version of the 555 timer using just NAND gates, opamps , a transistor and a few resistors! Now, you may think what is the purpose of building this when you can buy the IC at very cheap rates. The answer is, you learn electronics better, understand how the actual IC functions and improve your confidence in building electronic circuits.
More details on TheMagicSmoke blog.