[1] Ultimate Tag Warrior 3

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 [2] 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 [3] increase your Google PageRank as you go.