I found this buried deep in an appendix of the Mapnik XML Schema Reference, and I thought it so useful I am reposting it here:

Zoom level Scale denominator

0

559,082,264

1

279,541,132

2

139,770,566

3

69,885,283

4

34,942,642

5

17,471,321

6

8,735,660

7

4,367,830

8

2,183,915

9

1,091,958

10

545,979

11

272,989

12

136,495

13

68,247

14

34,124

15

17,062

16

8,531

17

4,265

18

2,133

19

1,066

20

533

These are used not only by Google Maps, but also by Bing Maps, OSM, CloudMade and many others, and in fact just about any Google Mercator tile source.