Avoiding duplicate page titles dinging your site's ranking
Having unique page titles, and other page metadata, is supposed to help search engines give useful information to users. Hence, Google wants to punish sites that don't do so.
Setting up Google Authorship linkages
Google is using rel=author and rel=publisher link attributes to link content to authors to provide improved search engine results.
Implementing metatags on Blogger, Drupal, etc
Metatags provide extra data which get used by search engines, social networks, etc, to present information on their sites as you want it presented.
Free (or not) keyword research tools
Understanding the phrases people use in searching for content can make the difference in being found, or not.
Semantic web and microformats
Page metadata is supposed to help search engines give useful information to users. Hence, it's something search engines are studying to use for web site indexing.
