Bitcoin Days Destroyed
Bitcoin Days Destroyed is a measure of the transaction volume of Bitcoin.  A bounty for a script to compute the Bitcoin Days Destroyed by the transactions in a block has been awarded.  ABE is a block chain browser that computes this statistic in real time.
If someone has 100BTC that they received a week ago and they spend it then 700 bitcoin days have been destroyed. If they take those 100BTC and send them to several addresses and then spend them then although the total transaction volume could be arbitrarily large the number of bitcoindays destroyed is still 700.