The Cthulhu Mythos have pretty much been a part of D&D since the beginning, though its official presence was largely scrubbed from it for legal reasons. As we are now well past the 70th anniversery of H.P. Lovecraft's death, and there are no well monied conglomerates claiming his works as their own (as with Robert Howard and Conan), his stuff is now generally considered public domain and thus is getting more and more used.
Realms of Crawling Chaos (RoCC) is such a supplement for Labyrinth Lord (LL), on adding the Cthulhu Mythos to the game. If you are unfamliar with it, LL is a retro clone for Basic (by Moldvay) and Expert D&D (by Cook), commonly referred to B/X D&D or just B/X. LL also features an advanced companion, which more or less emulates 1st Edition AD&D. This supplement supports both regular LL and advanced, so should work with pretty much any old school D&D based game.
Although a fairly slim tome at 66 pages, RoCC is reasonably comprehensive. It starts off (at least rules wise) with Lovecraftian character races, those with the Deep One taint, Subhumans (half-Voormis, which was really more Clark Ashton Smith)as well as White Apes, such as described in the story Arthur Jermyn. Rules for racial classes (as in B/X) or for separate class and (OD&D/AD&D)race are provided.
Next is a small section of spells taken from HPL's writings. They are presented more or less as normal D&D style spells, which is really not the case from the stories. For instance, in Charles Dexter Ward, the doctor learned the reverse of the "Revivify from Essential Saltes" spell in a few hours. In the book, it's a 5th level MU spell. Similarly, Voorish Sign is a 5th level MU spell, had Wilbur Whately been a 9th level MU (he could use the Voorish sign), he probably wouldn't have been killed by a guard dog.
By and large it works, but a parallel magic system, like say rituals, that could be done by anyone probably would have made more sense and fit more with the stories.
Nearly 20 pages are devoted to monster stats for various Lovecraftian beasties, then various Great Old Ones. Elder Things, Great Race, Mi-Go, Deep Ones, all the ones you expect to find are here. Old Ones include the usual suspects like Cthulhu and Tsathoggua, but also Atlach-Nacha and Abhoth from CAS.
There's a very small section, only a few pages on various artifacts and gizmos, then a few pages on Psionics, which seems a little out of place, though I guess arguably some Lovecraftian entities had them.
The book ends with several appendices. One on reading Lovecraftian tomes, another on random effects of Lovecraftian magic items (usually quite icky), an appendix explaining where everything was derived from (which is helpful), and a section on how to integrate the psionics rules for Mutant Future (a Gamma World inspired game)
The Realms of Crawling Chaos probably isn't worth the $18 that the softcover goes for, but for $5, the PDF has good (if not exactly exceptional) value.