Kind of like eraser for pc, do they have something like this for servers? If somebody has confidental files on a server that you want to delete by over-writing is this possible? Would software like eraser work on a windows server? What about a linux server?
I don't think there should be any difference at all, between wiping hard drive on a desktop and server OS. In fact, Eraser's web page explicitly states, that the program works on Windows Server 2003 and Server 2008.
Servers and desktop both use a hard disk, it should be the same when erasing data. I use BCWipe for Unix servers, I think they have a free version somewhere, at least some Linux distro does include it.
There is a good and fairly recent article on "Howto Delete Files Permanently and Securely in Linux". A standard dd program, can also be used for that.