Floppies and other media imaged in the course of the data archiving hobby.
Wherever possible, disk images are in the format most used by emulators and imaging tools for that platform, EG, .D64 for Commodore 64, DMK for TRS-80, etc. If there is no standard image format for a platform, or I lack the tools to coerce a raw flux stream into a usable image format, zipped SCP flux files will be uploaded. For disks that use standard IBM sectors, usually I will upload IMD images as they provide the best balance of compatibility, preservation of original disk layout and format, and compactness of size.
Images are in directories by platform. Where possible, image files will be accompanied by file listings and extracted image contents in subdirectories. I like to preserve the original file formats, but will also convert to text where possible. Eventually I will get around to uploading scanned pictures of the disks themselves.
I would really like there to be a standard archival format that can store metadata, disk data, and a cover image all in one file, similar to the way Adobe Digital Negative works for pcitures...might look into creating that in my copious free time.