Google used to offer up an automatic definition from sources like Dictionary.com or Answers.com. Now there's a little blue "definition" link on the right side of any word or phrase search, offering Google's own homebrew definition answers.
You'll still see answers from Answers.com and other sources high up in the search results, of course, but Google's own definition link lays out a word's definition in traditional dictionary style, with usages, phonetic breakdowns, and multiple snippets from other web definitions. There's also a link for "Starred words", but I couldn't find a way to actually star the word you're currently looking at.
Is Google your good-enough dictionary these days, or do you find yourself liking the service of sites like Dictionary.com?
Google Replaces Answers.com with Google Dictionary [Google Operating System]