Billing itself as "The biggest dungeon ever published by the OSR. A campaign, a massive generator, and a grotesque setting all-in-one", Castle Gargantua would be better described as the best procedural dungeon generator ever published by the OSR, and possibly ever published in general. Its size is a little bit of a rhetorical sleight-of-hand or white lie, because what the author means is (as you may expect from the title) that Castle Gargantua is a castle for giants:
Castle Gargantua is about the same height as the Empire State Building—1,250 feet—and the same size as Ceausescu's Palatul Poporului in Bucharest, a little bit below four million square feet—floor area, the same size as the entire old city of Venice. Its rooms and corridors are so huge that clouds hover within them. Sometimes, it rains inside. There are miniature tornadoes in the spiral stairs and strong drafts of wind where the corridors are sloped. If a curtain fell, its weight alone would smash a dozen men to a pulp.
In other words, its size is physical rather than in terms of having a large number of rooms, which is what I instinctively think of if somebody calls something the "biggest dungeon" ever published.
That said, a slight mis-labelling is one of the few criticisms I can level at Castle Gargantua, which had me hooked by page 8, with its expertly-judged rumour tables, bespoke for each character class. As a procedural dungeon generator it can have few peers: it makes exactly the right choice, in my view, in providing a number of properly-mapped and keyed "special" encounter regions (done by the always-excellent Dyson Logos), but abstracting most of the rest of the process in such a way that allows the DM to create rooms and corridors off the cuff during play. This is a difficult thing to manage, but Castle Gargantua just about strikes the perfect balance between providing a certain amount of randomly-generated details without being too slow and cumbersome to use at the table. It does this through a clever snakes-and-ladders mechanism for generating dungeon regions, combined with colour-coding for tone and flavour, which is both as simple as it needs to be, and also (I think) completely novel.
One thing that hasn't often been mentioned in the reviews of Castle Gargantua is the implied setting, which, to me, has a certain fairy-tale vibe which you don't often come across in OSR products. Castle Gargantua's contents are grim and certainly for adults, but the works I was reminded of most while reading it were the Alice books, Labyrinth, Jack and the Beanstalk, the Gormenghast books, and the Wizard of Oz, if strained through a sieve of Moorcock and Howard. As well as being mechanically innovative it also struck me as being tonally refreshing, which is not to be underestimated given the lack of variety in that respect amongst DIY D&D fare.
Fire on the Velvet Horizon (http://www.lulu.com/shop/scrap-princess-and-patrick-stuart/fire-on-the-velvet-horizon/paperback/product-22094199.html)
Patrick S and Scrap Princess created what is, in essence, a paragon of the stat-less bestiary or the Uncle Andrew's bestiary: a group of monsters, described and painted in an unreliable-narrator-type fashion, for individual DMs to adapt and stat-up as they see fit. But describing it in that way doesn't do justice to the fact that these monsters are so good and so unusual.
It's very hard to think up genuinely new monsters. Either they are a riff off something which exists already within either D&D or mythology in general, or they are a riff off an existing animal or group of animals with powers which somebody has already thought of. It's very difficult to escape from the "this with a this" formula ("an orc with psionic powers", "a lion with the head of a snake", "a vampire with the ability to turn into a fox", "a big parrot with the powers of a blue dragon"), which is why it holds true for 99.9% of the monsters that anybody ever thinks up in the modern age.
Well, Fire on the Velvet Horizon for the most part manages to completely do away with the "this with a this" formula, and that is just about the highest praise I can possibly think of for a monster book.
It's also beautifully written and illustrated, which is the second highest praise I can possibly think of for a monster book or RPG product generally.
Vacant Ritual Assembly (http://redmoonmedicineshow.com/)
There has been an explosion of 'zines in the DIY D&D universe, and Vacant Ritual Assembly is at the forefront. The content is always useful and/or interesting, but what I like most about Vacant Ritual Assembly is what it signifies: if you want to do something, just do it. Create.
I also like its physicality. This is a 'zine which is meant to be bought and physically read, not downloaded and looked at on a screen - although it is available in PDF. Like most things in modern life, the PDF has always sacrificed usability for humanity, for soul, and when I pick up a copy of VRA and hold the printed pages - lovingly but slightly-amateurishly put together - I feel a sense of communion with the different writers and artists that I never would if viewing their work through a screen. This is what a lot of the joy of traditional RPGs is about, for me: they are like wood. VRA is like wood, and I mean that in the nicest possible way.