Of course, it's relatively easy to create a customized error page. bMighty has one, for example. But while our error page offers instructions on how to get to many things bMighty.com readers might be looking for, it's not customized to match what users might actually be seeking.
Best of all, while the Toolkit defaults to Microsoft's Live Search, you can specify it to use any search engine you like, as well as return results for the specified domain and locale, control the number of results returned on your page, choose whether to offer spelling corrections, and customize your error message.
Given that it's estimated that up to 10% of Web traffic turns out to be error pages, it wouldn't exactly be a mistake to implement this on your company's sites.