Google Webmaster Tools
I've never really played with this in any depth before since it was just "Google Sitemaps". After looking over it today, I decided that should change. Here's why:
- Detailed reports of Googlebot crawling your site. See which pages it had trouble with and any errors encountered.
- Analysis of your pages - including the words that Google finds to be most common (useful when SEOing) AND the most common words that link to your site!
- Search queries that visitors used to find your site, including your ranking for that term.
- Ability to view internal and external links to your site - more detailed than the link: command, with a "download to csv" button.
- A feature to change how quickly Googlebot will crawl your site, with stats on how long it currently takes to request one of your pages.
In conclusion, if you havn't tried it already, do it now 
Comment by SarahG — February 7, 2007 @ 12:07 am
The Webmaster Tools is a pretty handy app, and even better now they added the external links in.
Yahoo has a basic version at http://siteexplorer.search.yahoo.com/ however it only shows you how many pages, inlinks and subdomains Yahoo has indexed at present.
Lots of good info at the Google one however. Even this morning I noticed their spiders were picking up some 404s on a site so I'll be fixing them when I get a chance
Comment by Matt C — February 8, 2007 @ 3:19 pm
I just signed in to my Google acount and added a site. Looks quite good.