Ask HN: Options for website and application analytics

I'm currently thinking about how I can measure audience metrics for a new project.

For those of you who work with such tools, how do you handle product analytics? Is everyone using Google Analytics? Are any of you hand rolling solutions? And if you are, do you have any regrets (the common ones I can imagine are fighting for disc space, and visualization)?

And what opensource tools are worth looking into?

Most of the opensource projects I've seen are SQL based. Do any of them allow pluggable backends?

Any other insights will be most welcome, Thank you.