January - iGoogle has a new theme directory, where people can upload interesting themes. Later this year, Google uses three collections of iGoogle themes (artist themes and non-profit organizations) to promote the personalized homepage and Gmail also gets themes.
February - a Russian ad recreates Gmail's interface in real life.
March - Google's homepage has a black background to promote Earth Hour, an event that encourages energy conservation.
April - Google's April Fools' Day hoaxes are more numerous than ever. From human colonies on Mars, human-powered search, rickrolled YouTube users to searching the web pages published tomorrow, there's something for everyone.
May - Google Translate adds 10 new languages, including the very difficult Finnish.
June - 2008 was a great year for Blogger, which added many useful features, including an inline comment form and an option to export blogs.
July - Google launches Knol, a service that encourages people to share knowledge. Built on JotSpot's wiki technology, Knol's goal is increase the amount of search results that offer high-quality information. "There are millions of people who possess useful knowledge that they would love to share, and there are billions of people who can benefit from it. We believe that many do not share that knowledge today simply because it is not easy enough to do that. The challenge posed to us by Larry, Sergey and Eric was to find a way to help people share their knowledge," explained a Google blog post.
August - Google Search Insights is a more advanced Google Trends for advertisers and enterprises. Use it to find popular queries in a region, to restrict the data to a category or to monitor interest over time.
October - T-Mobile G1, the first Android-based phone, is now available. Crunchgear thinks that "the hardware design is dated", Engadget is optimistic about Android and its app marketplace, while Laptopmap thinks that the interface "is perfectly functional but lacks the polish and consistency of the iPhone's".
November - SearchWiki is a new feature for Google search that lets you highlight search results and annotate them. While the added functionality is certainly useful, Google could integrate in a less cluttered interface.
December - Gmail Tasks is the 31st experimental feature added to Gmail Labs. Adding a task management service should've been a higher priority task for Google, which still has a lot to catchup with more mature services like Remember the Milk.
Related: Also see the 12 images for 2006 and 2007.