Considering the age of Doom now, it's some kind of a miracle to me that there are still new people arriving at the modding communites and starting to work on their own projects. Not that it is a bad thing, it's quite the opposite as those people get some new ideas into this fucking old egoshooter. Though, starting off from zero is a hard thing considering all the different applications and programs you can work with. This article is supposed to be a collection of tools that are a useful base if you want to work effectively on a new ZDoom/GZDoom project without having too much trouble or spending too much time on looking for them. I will also try to keep this up-to-date as soon as new programs are availabe, so just to let you know that this isn't outdated in the next few weeks So, let us not waste even more time and read on!

The Map Editor
"Easy and fast - it's DoomBuilder 2"
Since almost ten years, I am an active mapper in the community and therefore I had the chance (or out of necessity) to use a large variety of different editors, like WinDeu, Deth, Zeth, WadAuthor and Zeth but none of them has been as advanced, as fast and as easy to learn as DoomBuilder and the all new DoomBuilder 2. The reason for this is definitely CodeImp, the creator of DoomBuilder. He is still actively working on improvements for his masterpiece mapeditor and what's even more important: He is working together with this on members from the community in terms of beta tests, bug tracking and feature suggestions. So, in a certain way, the new Doom Builder is a coproduction that aims exactly at the needs of us mappers. Additionally it's always up to date in terms of ZDoom/GZDoom compatibility (e.g. reading resources PK3s), and with it's countless possibilites and features, it's the primary choice for every Doom mapper out there. Nothing is better, nothing is faster, nothing is easier than Doombuilder. That's no hint, it's an assignment By the way, if you are more into next-generation-editing with all those nice dynamic lights and 3d models that GZDoom supports, you might also be interested in GZDoombuilder, a new fork that makes mapping even easer (thanks to UDMF btw).
Download
DoomBuilder 1 | DoomBuilder 2 | GZDoomBuilder
The Lump Editor / Organizer
"The choice is easy, it's SLADE 3"
There have been several tools created over the years to work with Doom's data formats. DeuTex, WinTex, NWT, XWE, etc. The only one that is still maintained and that is updated to handle the new features of modern source ports is SLADE 3. Furthermore, it also features online documentation with a dedicated wiki. SLADE features a simple and intuitive interface, can open PK3 files, edit PNG offsets, and handle many data formats including those exclusive to some source ports or used by other games. SLADE 3 also supercedes SLumpEd entirely, having all the features of its predecessors but a lot less bugs.

Download
SLADE 3 | SLumpEd
The DECORATer
"DecX - It doesn't only extract"
As of now, there is one single tool that can help you with all kinds of DECORATE needs that you could come up with. It's called DecX, standing for Decorate Extractor but in this case, that's only an understatement. DecX is a great decorate editor with all kinds of nifty features like syntax highlighting for actor and weapon codepointers, general decorate code and it is also capable of directly testing actors in a testing environment (weapons & monsters). For newbies, this is the perfect program to work and learn with. Unfortunately, it has some limitations. Nya didn't work on this for a long time now, so the syntax highlighting and possible codeointers isn't up to date and that's somehow a problem. The only way to avoid this, is Cutman Mike's latest application (yes, the man can do more than Ghouls ) DECORATE Assistant. The application simply pastes code for action functions and actor flags with the parameters ready to be changed, you only need to click on the system tray icon and choose the function you need and it appears in your editor or your clipboard if you prefer that. These two programs in combination and all your wishes are fulfilled.
Download
DecX 2 | DECORATE Assistant
And that's all you need I guess Sure, that's my personal preference and it's possible that other Doomers recommend other tools or even some more programs, but for me the ones that I've mentioned have been enough to create the Ultimate Torment & Torture, City of the Damned : Apocalypse and my ZPack submissions. So I think if you have these on your hd, most of the aspects you will encounter in mapping and modding for Doom will be covered.