When I first started doing Doom modding, I was pretty bad at creating custom resources. I'm still pretty bad, but I do have some stuff floating around that I would like to share. Please be patient- it'll take me a few weeks to get what I have up here.
I see a lot of ripped and edited content, and a distinct lack of explicit licenses in this area. While I'm not against such things, it makes it difficult to find resources that are usable for certain projects. I've decided to specialize in libre resources that are free to use and have full license and attribution information. That doesn't mean I won't post other content, but that's what I'm going to focus on. I'm not a great artist, so a lot of it will be edits of free assets, modified to work for Doom.
Libre Resources
These resources may be freely reused with a minimum of restrictions.
Keycards

License: CC0 (Public Domain)
Base: public domain images
Author: XCVG
A set of keycards, or credit cards, or ID cards, or whatever. The download below is for the Doom pack, which includes a set of Doom-paletted keys and a set of Strife-paletted keys. Raw PSDs and larger general purpose versions can be found at OpenGameArt.
Download | OpenGameArt
Night Skyboxes

License: CC-BY 3.0 (Attribution)
Base: Night Skyboxes by Emil Persson(Humus)
Author: XCVG and Emil Persson(Humus)
A set of five skyboxes scaled and packaged for use in GZDoom. Includes GLDEFS lump.
Download
War on Water Guns

License: CC0 (Public Domain)
Base: War on Water GFX (yd, various)
Author: XCVG, yd and unknown
Two ready-to-use guns as well as source images, including a chaingun sprite. Not the best, but completely free to use.
Download
Trees

License: CC0 (Public Domain)
Base: Glitch
Author: XCVG, Tiny Speck
Several trees from the abandoned Glitch project. Scaled, with DECORATE, but not paletted.
Download
Outdoor Flats

License: CC0 (Public Domain)
Base: various
Author: XCVG, various
Flats made from CC0 textures. Higher-resolution 128x128 with TEXTURES file included. I started with a bunch of outdoor textures and decided to go with that.
Download
64x64 flats

License: CC0 (Public Domain)
Base: 64x textures an Overlays
Author: XCVG, FacadeGaikan
A CC0 64x64 texture pack from OpenGameArt turned into ready to go flats. Includes truecolor PNG as well as Doom and Hexen palette flats. Two of the flats are animated and ANIMDEFS is provided for them.
Download
Paint and Solid Colors

License: CC0 (Public Domain)
Base: own work and public domain texture
Author: XCVG
A pack of solid colors and paint textures. Good for prototyping or maybe even in finished maps? There are more colours than shown in the preview.
Download
Gratis Resources
These resources are free to download, but they may be subject to significant usage restrictions.