CMOS square wave oscillator
Oscillator frequency is determined by the RC network and the propagation time of the inverters (in this case NAND gates). For this simple example and short explanation we will ignore the propagation delay, and just say the maximum frequency is 1MHz. It just has to be low enough that the propagation delay and other variables has as little effect as possible upon the oscillator frequency.
I like to use 4000-series CMOS ICs. These have very wide voltage range and low current consumption. IC used here is CD4011, and both buffered and unbuffered versions can be used. This oscillator will work between 5 and 15 volts supply. If you want to fine tune the frequency you can use smaller value potentiometer and larger fixed resistor, but the minimum value is 22k.
You can use this simple circuit as a clock generator for many different projects. It has CMOS level output and it's easy to convert it to TTL level for example arduino use or other TTL circuits.