Recently I've heard of a site which gathers prices from some exchanges, and gives you a mean value.
I've tried googling for it and I failed, couldn't find anything related for some reason.
Could you point me out to such a site?
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Probably this is more informative and partly older than BitcoinAverage:
Bitcoincharts provides financial and technical data related to the Bitcoin network. You can also access this data via our JSON-API.
Bitcoincharts uses daily intervals (UTC) to display data. For example, the volume of a market will usually be zero at midnight (UTC) and increase during the day.