I think it is a custom font so it isn't easy to duplicate exactly. I've found Arial (serif) is close though.

Why would you want to match the AdSense font?

Are you conducting a study of user confusion, conversion tracking, and Google's Smart Pricing algorithm? :-)

I've noticed it varies depending which type of ad format is used with an Arial style and Verdana style in 10 or 12 pixels.

font-family:verdana,arial,sans-serif

Directly from the source code. If you don't understand it means verdana first, if verdana isn't availiable arial is used, then if arial isn't there, it's sans-serif.

Thanks everyone.