If you have a search box on your blog, and want to keep records of what people are searching for, WordPress Search Meter will keep records of all search queries. It will keep record day, week and moth wise. Search Meter plugin has easy to use interface so it could be control and manage easily.

With Search Meter, you’ll be able to find out what people are searching for, and give them what they want by creating new posts on those topics. If people search your blog and get no results, they’ll probably go elsewhere. Search Meter also shows you which searches have been unsuccessful.

You can also show your readers what are the most popular searches. The Popular Searches widget displays a configurable list of recent popular successful search terms on your blog, with each term hyper-linked to the actual search results. There’s also a Recent Searches widget, which simply displays the most recent searches. If you are easy with editing your theme, both of these functions are also available as template tags.

You can get this plugin form here