This is the simplest circuit capable of alternately flashing two LEDs. The flash-rate is determined by the two 10k resistors and two 100u electrolytics. The flash-rate can be increased or decreased and one LED can be made to flash brighter by altering the 4 components.
The operation of the circuit is fully discussed in the Basic Electronics Course P17 & P19 (subscription needed) and in 5 Projects P16. (use this link for the web: 5 Projects)
It can be purchased as one of the kits in 5 Projects. (use this link for the web: 5 Projects)
This circuit does not have the advantage of the two described above. It does not pulse the LED with a high current for a short period of time and thus it is not as efficient as the two above. When a LED is pulsed with a high current for a very short period of time, the output is almost as bright as a constant, lower-current.
THE FLIP FLOP
Here are some Flashing LED circuits from "200 Transistor Circuits' eBook. The first 4 circuits show how to change an NPN transistor for PNP.