It's a Doom Editor
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 - it combines the features of both, to create an all-in-one editor. Why does it keep the name of what was previously just a map editor? Because it fits :)
Here is a brief list of features implemented in the current version of SLADE:
Basic archive/resource editing
All the basic resource management features you'd expect: Create/Open/Save archives, import/export, ordering, rename, etc.
Simple tabbed interface
Open multiple archives at once and switch between them easily via tabs. This enables simple copy/paste from one archive to another, among many other benefits.
Many supported game formats
In addition to full Doom WAD and ZIP/PK3 support, many other game formats are supported, including:
Advanced text editor
The SLADE3 text editor can recognise many text based languages, and provide syntax hilighting, function calltips and autocomplete for them. It also has a bunch of other useful text editing features such as autoindent and find / replace.
Graphic conversion
All supported graphic formats can be converted to PNG/DoomGfx/DoomFlat and more, with an advanced graphic conversion dialog that provides conversion options and previews.
Texture editing
Edit Doom composite textures (TEXTUREx) with the easy-to-use SLADE3 texture editor. Also fully supports ZDoom's enhanced composite texture format (TEXTURES).
If you wish to help support SLADE development, feel free to make a small donation, though by no means is this a requirement for continued development. I work on SLADE in my free time as a hobby project and generally enjoy it, which is enough for me.
Additionally, any help on the coding side of things is greatly appreciated - take a look at the list of issues, and feel free to submit a pull request.