How to setup WebObjects 5.1 on Linux describes how to get WebObjects (the original from Apple) running under Linux - due to the Java basis of the 5.x versions, this has become significantly easier by now. Although I am naturally much more interested in the Objective-C version for Linux - Java is not the big performer on Linux in terms of performance ...
Well, for the Objective-C version, you can also fall back on SOPE (I reported), it is compatible in many areas. For using XCode, there are also project templates, so you get similar comfort during development. I have to try how it looks with ProjectBuilder.