@pr0fess0r cha0s
For an editor, you can use virtually any text editor for DBC. You just have to call the DB command line arguments to run the source or convert it into an exe. You can find details about the command line options in the help folder in the dark basic directory on your hard drive. Look for the file compiler.html
For more IDE like editors that allow sysntax highlighting, macros, color selection, hex editing/viewing, I like Source Edit for any large projects. You can find discussion on this and other editors in the following:
IDE discussion
I had looked at NotePad++ in the past but can't remeber why I didn't download it. It looks quite promising so take a look:
Notepad++
As far as level designers, Robert The Robot put together a couple of nice apps: Lightning Limbs and Lightning Lights. Lightning Limbs is an animator for DBC. Lightning lights is a scene creator/light setup utility for DBC.
A pretty straight forward level editor/3d Modeler is Deled 3d. The lite version is free and extremely easy to use. It imports and exports models in the .X format for use with DBC. I haven't used Deled 3D for some time, so it's probably evolved beyond a world creator; but it's original function was quick 3D level design.
There's a program written for Blitzbasic called Maplet. It's a level designer that will export the level to .X format. It's sort of old school in that it uses only primatives (cubes, sphere's, etc) to design environments and uses CSG to carve shapes from two or more objects. It even allows lightmapping but you have to do a little juggling to get it to work with your .X exports.
Blender is my modeler/animator of choice though texturing well is still a mystery to me! This is a heavy hitter and takes time and use to make things happen.
Follow this link for a couple of interesting animations that I did in Blender and converted for use with DBC (Oh how vain I am!

)
Animations
Enjoy your day.