Ultimate Tag Warrior is a multi-headed hydra of a plugin. It allows you to add tags either through the Write Post page in WordPress in a tag box, on posts using an AJAXy box, and in posts using special syntax from external editors […]
Once you’ve got your tags entered; you enter a realm of interesting ways of doing things with your tags. […] At the simpler end, you can automagically have tags included at the top and bottom of your posts without making changes to themes.
Think about coupling this with the Related Posts plugin to get a really powerful tool.
But… if you don’t mess with links or change Permalinks on a daily basis you shouldn’t be having 404′s in the first place, should you?
And if you do need to change Permalinks or update the page/post slugs, see how you can actually benefit from it and increase your Google PageRank as you go.