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.

**Update 2012-02-02 17:57 -0800:I just wrote a post about another way to do this.

I really like QGIS. It’s a powerful GIS that runs on Linux (among other operating systems) and doesn’t require incredible amounts of CPU. One criticism I had of it was that it did not provide the same ‘click-and-go’ basemap experience one can get with ArcGIS for making simple maps (i.e. Add Basemap, select Bing, DeLorme, &c. and you’re done). No more! Thanks to the qgis-openlayers plugin available on GitHub, you can now use OSM, Google Maps, Yahoo Maps and Bing Maps as base layers in your project (side note: make sure you don’t violate any copyrights by using them). I’ve had a little trouble with the projections, but I’m not that good with dynamic reprojection in QGIS yet, so I’m sure it’s my error.; Unfortunately, the layers don’t support reprojection, so your project is basically forced to use Google Mercator—which may be a showstopper but is often acceptable for quick maps (remember, QGIS can reproject your other layers). If you do want reprojection, see this post.