Google has released (as a beta, under Google Labs) Google Suggest, which is really cool. You start typing a query, and you get the most popular matching queries in a pseudo-combobox. This is cool, because it allows you to see the most popular searches starting with the letters you typed. So, for example, you know that Firefox is more popular a search than, say, free.
You can also get the most popular RFC documents. The most searched for, as it turns out, is RFC 1918 - Address Allocation for Private Internets. I know that, because I typed "RFC " (without quotes, with the extra space), and it came first.