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.

ProThoughts.com

Simple redirect WordPress plugin, all 404 error pages are redirected to homepage. Google may lower PageRank of site if there are 404 error pages(Page not found), this plugin will fix that issue and help to improve your sites ranking.

404 ReDirector vs Redirect 404 Error Page to Homepage