Downloaded the latest version of the <oXygen> XML IDE last night and was impressed with what I saw. <oXygen> is cross platform and supports both Linux and Windows as well as the Eclipse IDE. One things that I have always like about the Altova XML Spy suite (that they remove from their community edition) is the ability to collapse an element in the edit interface. This comes in really handy when working with complex documents. <oXygen> also has a nice Schema interface, XPATH 2.0 interface and diffing tool.
Altova charges big bucks for all of this functionality. The equivalent functionality in <oXygen> for 1/4 the price, cross platform to boot? Summary good deal!
NOTE: If you are running <oXygen> under Linux you do need a Sun JVM. The JVM that comes with the Fedora Core 4 & 5 distributions is not supported. To make it work in Fedora Core, rename the supplied /usr/bin/java binary to java.old and then include a link called java to the SUN JVM.