Relationship between Wikimedia and Google.
Wikipedia frequently appears as one of the top Google search results. In fact, in 2006 Wikipedia was apparently the top recipient of Google Search's outgoing traffic. Google is also the top incoming external referer (sic) to Wikimedia sites. Partially because of Wikipedia's high PageRank score, it has become targeted by SEO / advertisements. In July/August 2015, Wikipedia saw a massive decline in page views, leading some to speculate that it had been penalized by Google. Business Insider attributed this drop to Google changing its algorithm to prefer official websites over Wikipedia. This has been contested by Wikimedia analytics, who noted that there was actually an increase in viewers from Google. Stone Temple Consulting compared rankings for Wikipedia and Google's own websites, and found that while Wikipedia's ranking has slightly decreased, it is still higher than Google's own properties.
Google Knowledge Graph
In 2012, Google introduced a new feature to its search engine called Knowledge Graph. This summarizes information from around the Web about the search term in a box, similar to an infobox. According to various news sources, this feature has caused a decline in Wikipedia's page views.
In 2010, Google donated USD$2 million to the Wikimedia Foundation. Google also funded Wikidata development with a "generous donation". Also: wmf:Press releases/Brin Wojcicki Foundation Announces $500,000 Grant to Wikimedia
As noted above, Google funded development of Wikidata. They also announced that they were closing Freebase (which is used by Knowledge Vault and Knowledge Graph), and merging its content into Wikidata.
Google Summer of Code