From Transparency International, the Corruption Perceptions Index

Corruption Perceptions Index. Where there is smoke, they argue, there is fire.

Here's a link to documents for the 2007 version.
Here's a direct link to the Index.

From the website of Transparency International:

The 2007 Corruption Perceptions Index looks at perceptions of public sector corruption in 180 countries and territories - the greatest country coverage of any CPI to date – and is a composite index that draws on 14 expert opinion surveys. It scores countries on a scale from zero to ten, with zero indicating high levels of perceived corruption and ten indicating low levels of perceived corruption.