Global Historical Climatology Network
NOAA Global Historical Climatology Network is a collection of daily weather data put together from multiple weather services around the world, even those where at this moment no data is publicly offered. Resolution is fixed on daily, because this is the most common and maintainable resolution with most observations practiced all over the world.
The NOAA GHCN incorporates data of multiple weather services across the globe. The NOAA GHCN description page includes more information on the usage, references, qualities etc.