brainfruit

This simple plugin – which is also WordPress network compatible – automatically redirects 404 errors to the best matching and existing content in your WordPress blog. 
The matching is based on the slug of the content not found and searches also on taxonomy pages (tags & categories). This is perfect when you have changed the permalink settings of your taxonomies. 
All redirects are permanent (301) which is a SEO best practice. 
Since this plugin doesn’t need any configuration it works perfect in WordPress Network / multisite setup.

404 ReDirector

Simple, SEO friendly, permanent (301) and automatic redirect of 404 errors.

The plugin allows you to temporarily or permanently filter certain words from the 
title, content, excerpt or tags in posts.
You can choose where to search exactly or select everywhere in the posts. The 
original content of posts is kept safe. You can always go back and show the 
original text.

GSY Content Filter

Filter words from the title, content, excerpt or tags in posts.

404 ReDirector vs GSY Content Filter