NWT (New Wad Tool)
This is a small DOS-based .wad editing tool for Doom & Heretic. Features include image and sound previewing, intuitive interface, ability to import, export, add, and delete resources, .wad cleaner (so your .wad takes up less disk space), wad joiner, and the ability to convert .wav, .voc, .pcx, and .gif files into raw resources for your wad. (Both come with limited support.)
This isn't exactly the best utility in the world for doing most tasks, mostly because it is very outdated, but it is fast, convenient, and easy to use. It can be downloaded here.