Perhaps the most simple circuit to make, the classic 555 IC is used to flash 2 LEDs on and off alternately.
Suggested values for components are:
R1 10k, R2 10k, C1 47µf
The resistors may be replaced with variable resistors to allow for fine tuning of flash rates
It is also possible to make the 555 switch a relay on and off, the diagram for this is shown below
