Well, here it is, the first Swords & Wizardry Appreciation Day. Perhaps this will be the only one, or perhaps there will be more. Certainly, a number of the people involved are making posts all week! I'm just doing the one, though, because I am a lazy blogger.
Of all the retroclones, Swords & Wizardry was the most interesting to me from the start. Where Labyrinth Lord and OSRIC presented the rules very closely to the source, S&W chose to make a couple of significant changes that streamlined play (and also moved the rule sets further from the possibility of the current D&D copyright holder choosing to engage in legal shenanigans to try and suppress the game). It turned out that the legal worries were not very important but, as often happens when constraints are placed on creativity, it resulted in some really interesting design choices that further illuminate what we mean when we talk about "D&D", and even roleplaying in general. For that alone this is an important set of rules. However, it is also an elegant and fluid set of rules that works well at the table.
Swords & Wizardry comes in three basic "flavors": Whitebox, Core, and Complete. Each of these represents a version of the original D&D rules set, with varying amounts of influence from the four supplementary booklets that were published for use with it. S&W:Whitebox sticks almost entirely to the original three booklets that came in a white box. S&W:Core adds material from the first supplement, Greyhawk. S&W:Complete adds more material from the supplements, and furthermore continues on to some of the early options presented in The Strategic Review and the first few issues of The Dragon magazine (before it lost its "The"). In addition to these basic "flavors", other people have put together versions that emulate the Holmes "Blue Book" edition of Basic D&D, that reskin the whole thing for Japanese-style fantasy adventures (Ruins & Ronin), and so on. In addition, there are a few supplements that alter the basics to better fit specific settings (my own favorite is Savage Swords of Athanor). The Whitebox and Core versions are published as POD products on Lulu.com by Mythmere Games, while the Complete version is published in print by Frog God Games. Over there on my blog's sidebar, there is a link to the online SRD for S&W:Complete, though. Also, Mythmere provides free versions in PDF and even .rtf format for those who want to use the text to write their own version.
Damage of all weapons, as in the original edition (and the Holmes Blue Book), is always a single d6, though a modifier of ±1 is given for heavy or light weapons. Armor Class, in a particularly interesting move, is given in both the traditional descending form and also in the innovative ascending form of the WotC editions of D&D. So, an Armor Class might be listed as 5, with the number in brackets being the ascending form. The license to use the rules does require that this format is always used for Armor Class, so derivative games need to do so. This ensures that the players and Referee will be able to choose which they prefer at the table.
Other than lists of equipment, spells, and monsters, and a system for placing treasure, that is it. A completely simple set of rules that avoids the sometimes odd emergent behavior and occasional outright missteps of later editions of D&D.
The somewhat longer S&W:Core adds a number of things from later supplements for the original D&D, such as an optional Thief class, variable weapon damage by weapon type, variable hit dice per class, and expanded spell listings up to seventh level for Clerics and ninth level for Magic-Users. There are a few spells that I'd use the Core versions over the Whitebox ones (Contact Other Plane, I'm looking at you), in part because they are actually closer to the original writeups in the three original booklets, and in no small part because they work better than the Whitebox versions.
In addition, Core explores hirelings in more detail, adds a rudimentary mass combat system, more monsters, and more treasures (especially increasing the number of magic items).
S&W:Complete finishes this by adding all of the fundamental classes that most of us got used to in AD&D, such as Rangers, Paladins, Druids, Monks, and Assassins.
For myself, I am especially attracted to the stripped-down, house-rule-ready version that is Swords & Wizardry: Whitebox. I am particularly attracted to stripping out (arguable) mistakes like the Thief class, as well as questionable "improvements" like variable hit dice and variable weapon damage. A house rule that gives secondary skills to characters whose "class" is simply a description of whether or not they use magic spells, such as the skills system in Savage Swords of Athanor, seems like a good way to avoid the need for a large number of character classes while also separating character class from the idea of a character's profession. Merging that with, for instance, the secondary skills system that is in Adventures Dark and Deep might be a fruitful way to go, actually.
Another useful item from Mythmere Games is the Swords & Wizardry Monster Book, which includes (according to the back cover) 265 monsters from the original books (0e-2e), 3 monsters from the third edition, 9 monsters from Monsters of Myth, and 186 monsters never before seen in print. Some of the entries are dull (it's difficult to make a wolf interesting, but it is something that needs to be statted out), some are interesting (the Wandering Hole!), and some are spectacular (such as, for me, the Tsalakians and Kzaddich, the Artificers of Yothri and their Amphorons, and the various Toad monsters such as the Yurmp and the Todawan Masters, among others).
Most of the books from Mythmere Games are available for free in electronic version, and none of the books is very expensive in print. The Frog God Games version is available online (again, the link is over there to the right), and is not too expensive in print either.