If you have installed Google analytics then you will automatically be accumulating information about your users in huge detail. Aside from all the usual analytics stuff such as how they found your site; how long they spent on it; and what pages they looked at; you also find out other more personal data. This includes where they are; what ISP they’re using; what browser they’re using and their screen size.
Others of you might not have installed Google analytics, but may well have a feedback form that asks for email addresses.
Good luck finding and using one – let us know how you get on using the feedback form below.