The government is publishing spending data on all transactions over £25,000 from today.
The coalition pledged to get the data up when it came into power, and it represents the latest step in their drive to make government more transparent.
All departments will publish details of their spending over £25,000 for the last six months on number10.gov.ouk and data.gov.uk.
The Open Knowledge Foundation is holding a conference today for the launch of the data, and to discuss what it might be used for.
Cabinet Office minister Francis Maude said, "We've already released a revolutionary amount of data over the last six months, from the salaries of the highest earning civil servants to organisation structure charts which give people a real insight into the workings of government and is already being used in new and innovative ways.
"We recognise that this is a work in progress, and some of the data we have released so far is not as detailed or as comprehensive as we would like but we are committed to making information as readily available and easy to use as possible."
The data will be published on a monthly basis from now on.