The one I use is Hammer. It has been tailured to edit or create Half Levels but it can be used to create your own.
Find Hammer at
http://collective.valve-erc.com/index.php?go=hammer
Although Hammer comes with a BSP, VIS and Light compiler, it is recommended to download the Zoner Halflife Compiler tools. You can obtain these from the same site as Hammer.
You will need a texture editor for creating WAD files ad Hammer will only import textures from WAD files. The one I use is Wally. You can also download Wally from:
http://www.telefragged.com/wally/
Using these two tools, you can generate exceptional BSP files.
Hammer is not the only editor available. There are others which may suite your requirements. Cartography shop can generate files specifically for DarkBASIC Professional. Check it out
"Man who looses key to woman's appartment...... He get no nookie" - A wise chinese man.