Google already provides this information via Google SiteMaps. I have used it for the past three years - works great.
Add your site to SiteMaps and put a generated SiteMap XML of your site (Google for websites that provide this free) on your web server, then let Google do the rest. There is section in SiteMaps called Crawl Stats that gives you what you want.
Get Google's view of your site and diagnose problems
See how Google crawls and indexes your
site and learn about specific problems
we're having accessing it.
Discover your link and query traffic
View, classify, and download
comprehensive data about internal and
external links to your site with new
link reporting tools. Find out which
Google search queries drive traffic to
your site, and see exactly how users
arrive there.
Share information about your site
Tell us about your pages with
Sitemaps: which ones are the most
important to you and how often they
change. You can also let us know how
you would like the URLs we index to
appear.