I've talked before about how to set up a tagging system for your blog using entry keywords. If you're using Movable Type 3.2, the Tagwire plugin can make this quite a bit easier. It treats your existing keywords as tags - if you've already been entering tag-like keywords for your entries, you don't have to do anything extra, just start using the template tags to display information about your tags.
Tagwire also integrates with MT-XSearch to easily create individual pages for each tag. On my other set up, I was using MT's built in search script with a few hacks and an alternate search template. Tagwire with MT-XSearch is the same idea, but much easier to implement. As a note, if you have multiple blogs, you will need to create the XSearch Tagwire template module for each one, which also means that you can completely customize it for each blog.
I had been using keywords on this blog for a long time to indicate related entries. In a way, they're really like a secondary set of categories. That's now reflected in the sidebar where the actual categories are listed under the heading "Tips by Type". The keyword/tags/secondary categories are now listed as "Tips by Area".
It's all about making it easier for visitors to find related content, keeping them on your site longer and hopefully helping them with what they're looking to do.