The Sygian Library is an adventure by Cavegirl, a particularly creative blogger and maker of gaming material. It's a randomly generated dungeon, and can be acquired here, for the reasonable price of 5$. Now, there are plenty of great free D&D content on the web, is this worth the money? The tl,dr of this review is yes, absolutely.
The module is well described by the author here, but in brief, the adventure is a "dungeon generator", where the dungeon is not the typical rocky tomb, but a gigantic, sprawling pan-dimensional library. This means that the adventure could be dropped in any setting and "work", as all you need is an opening into the Stygian library. Instead of mapping every hallway and room, the adventurers progress "deeper" in the library, with areas of interest being generated. After dealing with an area and what is within, the characters can press deeper, go back, or "branch" sideway into another area. One would imagine that the map the PCs are making is complex, but the map the players are using is abstracted. The library is not "stable", and upon a re-visit its structure will have changed.
The GM can roll in advance or on the fly if they so desire - I would recommend in advance, but if the players go somewhere unexpected it's not very hard to just start making rooms at random.
The good: The theme is very unified, intriguing and slightly spooky. There is a semi-abstract "progress" system, the premise being that the PCs are looking for a specific piece of information in the sprawling library; and said system works well. As they go deeper, they find clues and meet NPCs that may have useful tips on how to attain their goal.
The encounters and monsters are very interesting. Some are foes, but many can be negotiated with. The librarians in particular are an intriguing bunch, with different "factions" looking after different things. Some of the rooms are less interesting than others, but some are *very* intriguing, and the players - even though their PCs are there to find something specific, not "solve the mystery of the library" can't help but wonder at what the heck is going on.
The art is good - I wish there was some more, but I liked what was in there. The prose is quite good - this isn't high literature, but Cavegirl clearly write better than average. The text flows and is easily read, without being boring. The layout is good, with several entries giving you specific page numbers. Pages 13-15 are key to run the adventure/make rooms. There are also further tables at the back, to create random books, extraordinary books and treasure.
The bad: Randomly rolling for things can lead to repetition - while there are plenty of entries, the dice kept spitting out the same results for some reason. Frustrating. The system is not specified. I *think* it's supposed to be B/X, but the HP seems rather low? Nevertheless, you should be able to convert to whatever it is that you are using. I used Troika! I'll also note that it seems better suited for a theater of the mind type of play rather than inch by inch trap hunt or tactical combat - if you need maps for that, you'll have to make them yourself, and this module may not suit your needs.
Conclusion: This is an excellent adventure, with few flaws and many strenghts. It requires a bit of prep time (it's a good idea to read it its entirety in advance), but very little "fixing". In fact, this is one of the best "effort to enjoyment" ratio for a module I've ever seen. The setting is original, fun and is not "yet another dungeon". I was concerned a bit about the random generation nature, which can sometimes result in a random nonsense mishmash, but not at all, the theme is very solid, and the entries well thought out. I was told by several people that this was a good product, and they were right. Go support gifted OSR authors!