Similarweb

Similarweb estimates traffic, traffic sources and audience overlap for any domain, using panel data and modelling rather than direct measurement.

Its most useful outputs are relative: comparing two competitors, or seeing how a site's traffic mix shifts over time, is more reliable than the absolute visit numbers.

Accuracy varies with site size — estimates for large sites are reasonable, and for small sites they can be badly wrong. Treat figures for anything under moderate traffic with scepticism.