What do you do with your Entry page Exit link traffic?

What do you do with your 401, 403, 404, 500 traffic?

I see lots of sites that give exit link traffic to google or yahoo.
Do they do this because they think it boosts their relativity or just because they don't know where else to send it?

I see lots of sites that don't redirect their error traffic, or don't even attempt to monetize it.

do you take your 404 (page not found) traffic and repoint it to the original site to a page that can be found?
or do you give the surfer a page of links to choose from?
or do you route the surfer to some other site, or some other
banner page to upsell programs?

since 401 (unauthorized) and 403 (forbidden) traffic is usually in my experience, someone trying to do something they should not do on your site, do you attempt to monetize it with another program?
or do you re-route them to your entry page or a legal area of the site?
do you give them a warning page telling you know what they did?
do you send them to a top list to get a fresh set of surfers in return?

just curious how much other sites monitor and use this type of traffic, and to what detail do you filter it?