I recently got a "Server is currently unavailable or down for maintenance" every time that I tried to pull up a web page on my local computer. It took me a while to figure out what had happened.
As it turned out, I had installed Railo on my computer and then deleted it (deciding to run the express version instead). Apparently, I must have missed an uninstaller or something.
Anyway, Railo added a filter called "Resin" to IIS (found by right-clicking on the "Web Sites" folder in IIS and going to the "ISAPI Filters" tab. Once I removed Resin from the list of filters, everything worked fine.
It took me a while to figure out, so hopefully this saves someone else some time.