What is the Bitcoin spot price and how is it calculated? I was offered to buy Bitcoin for today's spot price (MtGox to be specific). So, I am wondering, what is the spot price and how is it calculated? Is there a standard tool to look it up?
Just like with other commodities, there can be multiple "spot price" numbers.   

With gold, for instance, there can (and usually will) be different price on the Comex exchange versus the price the at the auctions in London for local delivery there.

So when referencing the spot price, that is ambiguous until the specific market whose prices will be used are clearly defined.

Even then there is the issue of the the bid/ask spread at any point in time.  Since there is no regulations defining how spot price is computed, there is ambiguity between whether the highest bid offer should be the spot price, or the lowest ask offer.  Others might consider the value from last trade as being the spot price.  Another approach is to find the value exactly in the middle of the bid and ask and deem that as the spot price.

BitPay uses an algorithm that attempts to be fair in determining "spot price" by using multiple exchanges:


‎https://bitpay.com/bitcoin-exchange-rates


And Coindesk provides a proprietary spot price service where "spot price" also is computed using data from multiple exchanges:


http://www.coindesk.com/price
http://www.coindesk.com/coindesk-launches-proprietary-bitcoin-price-index/