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.

alekc

Sometimes you don’t want to show parent directories in permalink because they are not really necessary or because you know that in the future your child directory will change it’s parent.
For example: 
+Work in progress 
–Site 1 
–Site 2 
+Work Finished 
–Site 3
In this case Site 3 originally have been part of “Work in progress”, and on job done it has been moved under “Work Finished”.
So if you install this plugin your link will be transformed in this way:
domain.com/parent_category/child_directory/post_name => domain.com/child_directory/post_name 
domain.com/category/parent_category => domain.com/parent_category 
domain.com/category/parent_category/child_directory => domain.com/parent_category/child_directory

Remove Parents

Remove Parents is a tiny plugin which remove "category" and parent categories from permalink.

404 ReDirector vs Remove Parents