Site Optimization: use Google Suggest and Google Trends to determine top searches
Google Suggest suggests terms related to whatever keywords you type in, and lists the number of results for each one. According to the FAQ, it uses data about the overall popularity of various searches to help rank the refinements it offers.
Google Trends allows you to compare the relative popularity of two search terms. Type in two keywords or phrases, separated by commas, and it displays a graph of their relative popularity. A nice plus are the markers that show when news stories came out related to the terms you entered.