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Dictionary & Thesaurus enable you to look up a definition or synonyms for any word in your document from Merriam-Webster's Collegiate Dictionary. It's located in the tools menu, and it works for English only. Similarly, Encyclopedia lets you check out the description of any word from Encyclopedia Britannica.
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Another new options lets you search for images from within Google Docs. Highlight a word, click Tools -> Search... using Image Search, and you'll get the results from Google Image Search. Find an image you like? Just drag an drop it into you're document, and you're set.