Its that time of the year again, you know, when Google says its doing some more spring cleaning out of season. Urs Hölzle, senior vice president, Operations, and Google Fellow, shares the list of products Google is discontinuing, and their expiry dates.

Renewable Energy Cheaper than Coal (RE<C): This initiative was developed as an effort to drive down the cost of renewable energy, with an RE<C engineering team focused on researching improvements to solar power technology.

At this point, other institutions are better positioned, says Google, to take this research to the next level.

However, Google will continue work to generate cleaner, more efficient energy—including its on-campus efforts, procuring renewable energy for our data centers, making its data centers even more efficient and investing more than $850 million in renewable energy technologies.

Google Bookmarks Lists: This was an experimental feature for sharing bookmarks and collaborating with friends, and is going to end on December 19, 2011. All bookmarks within Lists will be retained and labeled for easier identification, while the rest of Google Bookmarks will function as usual. (As Lists was an English-only feature, non-English languages will be unaffected.)

Google Friend Connect: Friend Connect allowed webmasters to add social features to their sites by embedding a few snippets of code. Google is retiring the service for all non-Blogger sites on March 1, 2012.

Instead, Google is gently nudging  to create a Google+ page and place a Google+ badge on their sites, as an alternative.

Google Gears: Earlier this year, in March, Google waved goodbye (by the way, Wave is going off too) to the Gears browser extension for creating offline web applications and stopped supporting new browsers. On December 1, 2011, Gears-based Gmail and Calendar offline will stop working across all browsers, and later in December Gears will no longer be available for download.

This is part of its effort to help incorporate offline capabilities into HTML5, says Google. You can access Gmail, Calendar and Docs offline in Chrome.

Google Search Timeline: Google is removing this graph of historical results for a query. Users will be able to restrict any search to particular time periods using the refinement tools on the left-hand side of the search page. Additionally, users who wish to see graphs with historical trends for a web search can use google.com/trends or google.com/insights/search/ for data since 2004.

For more historical data, the “ngram viewer” in Google Books offers similar information.

Google Wave: Google had stopped development on Google Wave over a year ago. But as of January 31, 2012, Wave will become read-only and you won’t be able to create new ones. On April 30 Google will turn it off completely.

Users will be able to continue exporting individual waves using the existing PDF export feature until the Google Wave service is turned off. (If you’d like to continue using this technology, there are a number of open-source projects, including Apache Wave and Walkaround.)

Knol: We are not surprised to see Knol go; we were wondering about its future when Sergey Brin and his better half donated half a million dollars to the Wikimedia Foundation (See: Sergey Brin, Anne Award Half Million Dollars To Wikipedia).

Google launched Knol in 2007 to help improve Web content by enabling experts to collaborate on in-depth articles. It did not get a chance to get out of beta testing. Knol will work as usual until April 30, 2012, and users can download their knols to a file and/or migrate them to WordPress.com. From May 1 through October 1, 2012, knols will no longer be viewable, but can be downloaded and exported. After that time, Knol content will no longer be accessible.

Google says it has been working with Solvitor and Crowd Favorite to create Annotum, an open-source scholarly authoring and publishing platform based on WordPress (which is currently live).