A handy plugin for searching and replacing content within your blog. A perfect example is the moving of images. Rather than hunt through each post and edit the url for the image, this plugin takes the work right out of the task. Also could be handy for fixing posts that are causing validation errors.
While at Urban Giraffe, check out John’s other plugins, including disable wpautop, which turns off the auto formatting in posts.
Also, if you are interested in theme design, his four part series, Dissection of a WP Theme
is as useful as they get.