http://www.similarweb.com/ is very accurate. I didn't know about this. Thanks a lot :-)
FYI, we have an API at https://developer.similarweb.com/ with much of the data available in our PRO tool, and we're the only such service with mobile & desktop data, as well as coverage of websites in over 50 countries.
It's really nice to see us mentioned here on ycombinator, made my day.
I'm checking a domain where I can access Google Analytics stats for comparison, and it is pretty accurate.