Concept: “Half Person, half Lemming, shaggy fur and dirty robes. They stink of insanity and great heights.”
Content: An creature compelled to throw itself (and others) from cliffs, towers, etc.
Writing: Includes lore, stats, and rules for controlling PCs (and the aftereffects)
Art/design: Typographical variety differentiates textual components; illustration adds character and floofiness
Usability: Presented in multiple formats for accessibility and ease of use

Concept: “What happens when the lore of our world collides with the Dying World of MÖRK BORG?”
Content: The second volume of collected FÖLK-LORE Jam entries
Writing: Varies by author, see individual entries under the FÖLK-LORE Jam tag
Art/design: Varies by entry; single-page entries are well balanced in spreads
Usability: Varies by entry, but it’s all Mörk Borg—how tough could it be?

Concept: “It might be just what you've been looking for, if you need to organize your stuff and don't mind feeding a ravenous creature copious amounts of meat.”
Content: A guardian monster and potential pet in three shapes and sizes
Writing: Highly instructive and dryly hilarious
Art/design: Black and white text, clean-line images, and austere layout nicely replicate an instruction-manual feel; design on the care/feeding page is appropriately jolting
Usability: Includes safety precautions in English and German and a note for GMs 

Concept: “This adventure, on one hand, is a murder ballad in the tradition of german «Bänkelsang». It is also an album crawl set to the Judas Priest album «Turbo».”
Content: I think the concept pretty much sums it up, but there’s also a motorcycle fight
Writing: Includes a general overview, descriptions of characters and scenes, and instructions for traversing the adventure and conducting the motorcycle fight
Art/design: A traditional presentation that’s heavy on the text but does include some original illustrations
Usability: Linear and fairly simple to use; stat blocks for the motorcycle fight are particularly well organized in clear tables

Concept: “Two monsters […] some scenario seeds you can roll up and a small dungeon”
Content: See concept
Writing: Monster entries provide sufficient context while the dungeon is clear and concise
Art/design: Monsters are well organized with appropriate illustrations; dungeon is primarily typographical, which guides reading, but includes a map and a creepy crayfish-baby
Usability: A bit of a variety pack; straightforward and easy to navigate