SLADE3 is a modern editor for Doom-engine based games and source ports.
It has the ability to view, modify, and write many different game-specific
formats, and even convert between some of them, or from/to other generic
formats such as PNG.

SLADE3 can be considered a successor to both SLumpEd and SLADE.  Some of
its features:

  - Basic archive/resource editing (create/open/save, import/export)
  - Simple tabbed interface with copy/paste support
  - Many supported game formats (ZIP/PK3, Quake PAK/WAD2, etc.)

WWW: http://slade.mancubus.net/