Social Graph API, a great way to find public profiles and connections between people, will no longer be available. The service was used by Google to find your social connections and show social search results. Unfortunately, "the API isn't experiencing the kind of adoption we'd like, and is being deprecated as of today. It will be fully retired on April 20, 2012."
Google will also discontinue Urchin (the installable version of Google Analytics), Google Message Continuity (an enterprise service that allowed Microsoft Exchange customers to back up emails to Google's servers), Needlebase (a data management platform acquired from ITA Software) and Sky Map (an Android app developed "to show off the amazing capabilities of the sensors in the first-generation Android phones and offer a window into the sky"). Sky Map was open sourced and will be developed by Carnegie Mellon University students.
Google promises to take "a hard look at products that replicate other features, haven't achieved the promise we had hoped for or can't be properly integrated into the overall Google experience," so I'm sure we'll see other discontinued projects. From Orkut to Google Talk (the Windows software), from Patent Search to Google Bookmarks and Translator Toolkit, from iGoogle to Picasa, there are many projects that might be discontinued. Moving from standalone applications to platforms, Google gets rid of many tools that solved only one problem and tries to convince users to join all-encompassing services like Google+ or Google Apps.