What's a good square wave generator?

If you google for 555 square wave generator you'll get thousands of hits for circuits based on a 555 chip that produce a square wave. There's a square wave calculator here, which should allow you to experiment with the calculations.

Plus as an added bonus 555 chips are dirt cheap.

Or look at 556 chips which are basically two 555s on the same chip.

For a simple oscillator people often think immediately of a 555 timer IC. This ciruit is even simpler:

The 74HC1G14 is the single gate version of the more common 74HC14 in SOT-23 package.

Please note: the circuit you linked to uses a comparator, not an op-amp. You can use op-amps in comparator circuits but they aren't up to the job for various reasons: op-amps are optimized for amplification applications where the inputs are driven to the same voltage through feedback, and may take a long time to recover from saturation when their inputs zoom apart through positive feedback like in this circuit. A comparator will be faster and will do the right thing.

As for circuits: I'd either use a LM393 comparator or a 555 (hard to beat: lots of manufacturers and you can get it from Radio Shack or in high quantities from Digikey at 11c) or a 74xx123 (this one from TI is 16c in large quantity). The comparator will need a few more parts than the other two.