flash suggestions for Canon G15

I have a couple of DSLRs and numerous lenses, but sometimes, it's not the right environment to lug them around. So, I picked up a G15, which has a lot of controls, features that make the smaller size worth it, to me.

It's got a pop up flash, of course. But, with a hot shoe, I was curious about which on-camera flash you might suggest. I want a more versatile light than the built-in flash can provide.

The camera might end up looking smaller than the flash with anything larger than, say, a 270EXII, but then again, I'm not sure what flash options are available.

I would appreciate suggestions from those who also use on-camera flash with such a camera, e.g., G12, 15, 1X, etc.

Thanks.