A simple search for find strings in your database and replace the string.
This is an incredibly powerful plugin that easily allows you to make changes to your entire database. However, I have found that it causes the Redirect plugin to break leading to a huge manual effort to sort through my 301 redirects.
Contrary to what you might intuitively think, if you try to simply run a test search in the "Search In" (second portion of the plugin) area with nothing in the "Replace" field in this plugin, it won't simply return the results -- it will delete all the results (replace them with nothing)!
This opposes the function of the top portion of the plugin (as well as every other search feature in any program I've ever used) which can easily lead someone to not realize what will happen.
Moral of the story: make SURE you backup your database before using this :)
*TO THE WRITERS OF THIS PLUGIN: It would be a very advisable and EASY safety feature to implement a confirmation box when someone makes a search with the replace filed empty.
This plugin does a great job of straight string replacement in the WP database.
Unfortunately, some meta data, such as options, etc. are stored as serialized data... which means that if you do simple string replacements on them, they will probably break.
So, MAKE SURE YOU BACKUP YOUR DATABASE before using this plugin, and if you are uncertain, avoid doing search and replaces on meta data fields (meta_value).