Do you mean examples of good 404-pages? There is not too many, because everyone is trying not to show 404 at all. And because site design of 404 is not usually done for user experience, it just for look and brand, so everything you find when doing some googling is purely graphic design.
But you can try CNN. They are offering every categories, but nothing that is related to content that poor user tried to reach.
That would be something that you guys could do better than everyone else. It suites to company strategy, I think.
I didn’t ask sidebars. Big screens aren’t mainstream any more and every devs should put their working hours to totally somewhere else than bulding sidebars and widgets.
But how’s like every WordPress? Majority of e-commerces?