Are you looking for an editor for your own engine, or just something that has an editor built in? There is probably not many editors that can work directly with your own engine, since there is no easy way that it could possibly know what you are doing under the covers.

Perhaps you just want a modeling package for .X files? If so, I believe Milkshape3D has a .X exporter that you could try out. I haven't personally used it, but it may provide a low cost solution (not open source though...).

Otherwise, if you want a full solution with an engine and editor built in, you could always check out Unity3D. This is again not open source, but it is much more fully featured than most editors out there anyways...