I have been testing SmartErrors from CloudFlare for a couple of weeks now. It is active here on this blog. They have now officially launched it in beta.

CloudFlare SmartErrors is designed to replace your domain’s 404 error page with a search result page showing results connected to the keywords in the URL.

CloudFlare spoke about this new feature:

SmartErrors uses keywords from the referring link that produced the 404 to find relevant content and prevent your site’s visitors from using the back button and leaving your site to look elsewhere. In our testing across hundreds of CloudFlare sites we’ve measured average visitor engagement rates of over 12%, with some sites as high as 75-80%. What this means is that, on average, 12% of SmartErrors resulted in deeper engagement with a site. It really does make a difference!

It should work well if you site uses nice URLs with keywords in them.