Google Webmaster Tools is a free service that provides a good amount of information directly from Google.
Below are a few of the features with Google Webmaster Tools:
Google Webmaster Tools shows all sorts of errors within your site. Webmaster tools shows not only 404 pages, but unreachable urls as well. Also you will be able to see what urls are being restricted from crawling by the robots.txt file. This is good to know in case someone accidentally blocks too much.
2. Site Defaults
Tell Google to show your page with the www or without www, set a geographic
target and select if you want the images to show up in Google’s enhanced image search.
3. Analyze Meta Descriptions and Title Tags
Google will show a list of URLs that have duplicate title tags or
duplicate meta descriptions as well as if there are pages with too
short or too long meta descriptions or titles.
4. Top Search Queries
Webmaster tools will show the top search queries that Google is seeing for your domain.
5. Manage Sitelinks
If a site is fortunate enough to get sitelinks under your listing in Google, webmaster tools can help you manage the links