Evercookie's a great resource for dubious techniques to track people - look at the HTML canvas example for how it's done with a PNG:
https://github.com/samyk/evercookie/blob/6d44cb8ad1eda23c9a5...Luckily, the partitioning of browser caches on a per domain basis as pioneered by Safari has now made its way to Firefox (I think it's been released) and I presume will eventually be turned on by default in Chromium: https://www.chromestatus.com/feature/5730772021411840
