Over the past few years a number of WordPress plugins have been created to generate and display related posts for single pages of your blog. The original Related Posts plugin is still my favorite because it uses the post titles to determine which posts are related, but this plugin requires you to log in to CPanel and make some adjustments there (instructions on how to do this in the linked post above).
Another very comparable related post plugin is Related Posts 2.3, which was created after WordPress 2.3 added tags to the core software. In addition to matching up related posts by your tags, it also displays related posts in your feed (the first one I mentioned doesn’t do this on its own). This is of course only useful if you tag your posts responsibly, so it may not be a viable option for most people who treat tags as (meta) keywords.
If neither of these plugins fit your needs, there is a new plugin available which displays related posts by category. It does require WordPress 2.3 or higher to use.
According to the author of this new plugin:
The WordPress-Plugin Related Posts by Category lists similar posts within any article. As a search string the plugin does not use the title of the article nor weighs the content. In fact the category, which was assigned to the post, serves as the source of accordance. The reason for that: Posts from an equivalent category have most of the time identical topics and can therefore be seen as absolute relevant. Is an article assigned to more than one category, all of those categories will be used for the database query. Obviously this leads to more results.
This plugin requires the same responsibility as the tag one, though people are generally more disciplined with their categories than they are with tagging posts (because tags double as meta tags usually), hopefully making the resulting posts more relevant with this plugin.