Eclipse does allready has a
Junit plug-in installed.
For take a first view:
Right mouse click on a class.
Select new in context menu and then junit test case.
I haven't found info about usage in documentation which ships with eclipse.
It is pretty good explained in sourcebeat book by Bill Dudney, but there might be other usefull info on the web.
I'ld start with this one.
Axel