Dukascopy Bank provides extensive historical market data (forex, CFDs, metals, indices) often used by traders, researchers, and developers. Their data is valued for tick-level granularity and long time spans, but access methods, formats, and licensing vary and can affect usability.
If you require the full depth of market (DOM) or need aggregated data without the hassle of downloading 500GB of files manually, Dukascopy offers a "Big Data" service where they ship a pre-loaded hard drive or USB stick. dukascopy+historical+data
While tick data is the most granular, Dukascopy supports several other timeframes and formats to suit different technical requirements: indices) often used by traders