With ReReplacer you can replace whatever you want in your entire site.
It can work on the entire output of your site, so you can also do replacements in things like the meta tags.
You have the ability to use powerful regular expressions to search and replace. This opens up a endless possibilities. You can do virtually everything with ReReplacer
For documentation on regular expressions, see the
Regular Expressions Cheat Sheet
and Regular-Expressions.info.
RegExr is a great online tool for testing your regular expressions.
Because you can replace/remove all kinds of things, you can potentially break your site. It is not wise replacing words like 'html', 'body' or (parts of) words that are used in inline scripts.
However, the cool thing is: all replacements are not permanent. So when you turn off the plugin or remove the replacement, your website is back to the original state.
Also, there are some nifty features enabling you to replace stuff without replacing in fragile areas like in tags and outside the html body.