E621 DL https://pypi.org/project/e621-dl/
Download images from E621.net in batch, use the same tags of the site, remove the animated images with -animated to save space.
Krita
The best free image editor, i use to adjust images at multiples of 64 or 1200 X 1200, 1600 X 1200 (existing monitor resolutions).
Dupeguru by (https://dupeguru.voltaicideas.net/)
Remove duplicate images from your dataset, use it at 80% because theres nude, non-nude images some times (a piece of clothing missing), only duplicate a image if it's a good one and you need to fill a bit more of iamges.
img-txt_viewer by Nenotriple
Tag your dataset manually with side-to-side view (TXT and IMG) it can clean your captions and rename the dataset all at once.
Candy Machine by mikeknapp
Tag your datasets even without stablediffusion (A1111), ConfyUI on non-Nvidia GPUs
https://github.com/mikeknapp/candy-machine
I will add more tools, i use mostly the final training LoRA, but there is some graph based tools to check the learning levels of each Epoch.
Web Tools
Waifu2x
https://www.waifu2x.net/?lang=en
You need this to upscale some images to 2K,4k and extract some details from a big image (if you're hurting for more images take one and extract the face,upper body and full picture).
Cleanup
If you need to remove some details and don't have krita, it limits to 720P.
Frame Extractor
You can download a movie and put here to extract all the needed frames.