Google is typically very secretive about the specifics of its search revenue. I can't actually recall any other leak quite like this one, in which the budgets of specific companies are laid out—kudos to AdAge for snagging the internal document with such rarely seen information.
Much of the list, which covers the month of June 2010, will be of no surprise to anyone that uses Google Search regularly (which is pretty much everyone): AT&T spends ridiculous amounts of money, as do Apollo Group (which owns the University of Phoenix), Amazon, and Expedia. It's worthwhile to note that some of AT&T's $8.08 million budget was probably due to the launch of the wireless carrier's biggest product of the year, the Apple iPhone 4.
Apple itself spent slightly less than $1 million, which puts the company in the upper echelon of Google spending but not all that close to the top. 47 companies spent over $1 million, so Apple was, at best, in the top 50. That's indicative one of the more interesting revelations in the report: Google's search ads revenue is the product of dozens of different advertisers, none of whom dominate. The top ten advertisers only accounted for about 5% of Google's total revenue, and the biggest spender, AT&T, didn't even snag 1% by itself.
The big mover on the chart is BP. Normally barely a factor, spending about $57,000 per month at Google (out of a very large advertising budget—the company $94 million altogether in 2009), in June, after the Gulf oil spill became internationally infuriating news, BP spent a whopping $3.59 million at Google alone. As anyone who tried to read about the news can attest, BP bought a ton of ad space, including keywords like "oil," "spill," "leak," and "top kill."
Concerned citizens would probably have preferred that $3.59 million be spent on cleanup efforts.