An excerpt from http://www.sitepoint.com/managing-broken-links-404s-wordpress/, by @chrisburgess
Broken links create a horrible user experience. Who wants to click on
a link, only to find that it goes nowhere? That the awesome article you
were all set to read doesn’t actually exist?
Unfortunately, the natural decay of links (also often referred to as link rot)
occurs all too often. Link rot happens for any number of reasons:
domains expire, websites are abandoned, incorrect URLs are used, and
websites are restructured using new URLs. And, even though best practice
dictates the use of 301 redirects in the case of website restructures,
not everyone sets them up. Or, if they do – they’re often set up
incorrectly.
This shouldn’t stop you from linking to other sites. Linking to other
websites is an important component of what the web is all about. Some
people tend to think that linking out to third party websites will cause
visitors to leave their site, reducing their visitor stats, but linking
out is exactly what the web was designed for. You shouldn’t be scared
of it, rather, you should embrace the value that enriching your content
offers. Providing your visitors with related content or a deeper dive on
a given topic is a far better user experience than dumping users at a
dead end. This is just my opinion, I know there are different schools of
thought when it comes to the topic of outbound links.
So, if you can’t control other people’s websites, and you shouldn’t
stop linking out to other websites, then what can you do to make sure
you’re not sending people to non-existent pages? Well, it’s pretty
simple really: you can control your own site by performing regular link
maintenance and keeping your outbound links in check.
The management of broken links is an integral part of good WordPress maintenance.
And, thanks to a number of plugins and tools at our disposal, it’s
becoming increasingly easy to automate the process of link maintenance
these days. So, without further ado, let’s take a look at some of these
tools.