What real-life error web pages have you seen that have really impressed you AND would be genuinely useful to all types of users (savvy,novice,laid-back,frustrated,etc). Why were you impressed by the error?
Stackoverflow :) getting an error when using my claimID,...
the good thing I read on the error message/site was that this was NOT my fault, and its due to an internal error (and optional contact was there too)
why I think this is impressing?
there are thousands of sites, where you get an error and it just states "404" or "error, try again" - no word if I may doing something wrong or internal problems or, or...
Not exactly professional, but it gets the point across: http://www.thecenter2000.com/last/
Check out this page and you will know how to create proper 404 pages :)
The Firefox extension "Resurrect Pages". This replaces Firefox's standard error page, which is fairly informative but only marginally useful, with a page that is just as informative but also gives the user some tools to help work around the issue.
http://trac.arantius.com/wiki/Extensions/Resurrect#Screenshots
EDIT: Yes I know it's not a webpage itself, but there's no reason an actual webpage couldn't be at least this nice and helpful.
It's not just web pages, but About Face (Alan Cooper's book) has a good section on error message. Something like:
What happened. In laymans terms.
How does this effect the user. Did they lose data? Just an annoyance?
What they should do now (reboot, try again etc)
And put the techo info in a hidden area (ie, click here to get the propeller head version)
I did this for a project in WinForms and it worked well, tho we didn't get much feedback from the customers (tho they didn't moan either)