While the company has long used cookies, IP address logging and other methods of identifying visitors to its online properties, they are relatively coarse measures and easily defeated (eg, by blocking cookies altogether - a tall order these days - or at least by frequently clearing out the cookies).
Many of Google's more recent services and features encourage you to identify yourself to the company. Examples include Gmail (would you let someone else use your browser while you're logged in to your mail account) and the iGoogle home page.
The company's latest search improvement requires users to be signed in if they are to see any benefit.
Known as +1 (as in an online endorsement or thumbs-up), search results will be tagged with the names of people you know that have recommended that page. But - understandably - if you're not logged in, Google isn't sure who you are, and can't pass on the recommendations.
If you are signed in, how does Google know who you know? Find out on page 2.