404 pages are important. One of the biggest mistakes that blogs and website owners make is disregarding that fact and ignoring the 404 pages, thereby maybe losing all 404 incoming visitors. And that's a lot of visitors to lose.
If you're using WordPress, then to get a sense of just how many visitors you're losing, there's a wonderful MUST plugin called "Redirection" that aims at redirecting broken links into actual links. That's extremely useful, but for this post, this plugin also has an option for showing all the 404 visitors, where they came from, what page they were looking for. In some of the blogs I've suggested that this be installed, the 404 traffic was almost as high as the actual traffic. Even if you change nothing, things happen to a blogs - users post and send e-mails about your posts with broken links, search engines keep hitting the pages in which you've already changed the slug, stuff like that. Don't you wish you can turn all that traffic into actual readers? then I suggest you take care of your 404 page.
There are endless posts all over the blogosphere about what the perfect 404 page should include. I suggest you head out later and read "Page not found again! 404 in depth" that has some good suggestions. What I strongly recommend is using the Related Entries plugin with the 404 related-pages extension :
This plugin will only work if you had/have a permalink schema with the post titles in it. For example, /archives/2006/01/02/post-title or similar. In that case the plugin will strip out everything but the post title and use it to find entries that may be related to what the user may be looking for.
If you're wondering what my recommended 404 page is, then it goes something like this :
<h2>This is a 404 page.</h2>
<p>We're sorry, but we can't find the exact page you were looking for. This can happen for a number of reasons - a broken referral link, a page that doesn't exist anymore, or maybe even a temporary technical glitch. Either way, we'd hate to see you leave so soon, so while you're here, why not check out the following relevant posts: </p>
<p><?php related_posts_404(); ?></p>
<p>If not, you might want to go have a look at the <a href="<?php bloginfo('url'); ?>/">main blog page</a> and see if there might be something else that's interesting...</p>
<p>Either way, thanks for stopping by.</p>
Simple, straightforward, informative and relevant. It just might turn your random traffic into devoted readers...