Not likely the longest running server, by far, but I did manage to get a screenshot of a Lotus Domino server I updated recently before it was brought down for maintenance to install and upgrade to Domino r8.
177 days! Almost 6 months since it last came down.
This was the main web server too. I love it but who has a longer time frame? We shouldn't because we should be updating it on quarterly releases but still, solid as a rock.
Any Exchange people out there got one like this?