H.P. Lovecraft, secure in the safety of his home in Providence nearly a hundred years ago, might have shuddered in horror himself at the prospect of a future where the cyclopian architecture and creatures from beyond space and time would be part of a large game played by eager fans. The horror of this future is paralleled only by the fact that he could have used those royalty checks, even then.
On a slightly more serious note, I would like to take a moment to talk about one of my favorite roleplayng games, Chaosium Call of Cthulu 6th edition. Now, the people who know me, especially those in the currently running Call of Cthulu 1920's Arkham campaign so ably run by BJ Pehush, know that up until about 3 years ago I was firmly a d20 man. Every system had to be a d20 product, with rules that I could easily remember from the Wizards of the Coast d20 System Reference Document. Then I tried Call of Cthulu d20... and I absolutely hated it. I came to the realization at that point that while I love d20's for Fantasy systems, modern and science fiction games really just don't translate well. Thus when Chuck's D&D/Space 1889 combo campaign came to a close, and BJ announced an interest in running Call of Cthulu, I suggested using the Chaosium ruleset, and thankfully he agreed.
What makes the Chaosium system different, though? Well, for starters it is a percentile system as opposed to a flat d20 system. Before you get your hackles up, though, I would advise you to keep an open mind. The system uses percentages for all skills, adding in additional percentage points as you succeed in skill challenges. In addition, only a few of the base statistics get used, and they generally correspond to a related percentile (Luck, Power, Sanity, etc).
What does this mean for the player? First it means a simpler way of character improvement. With no levels, no big power ups, no increases in hit points, no feats, no talents, no skills, etc, the player is really only worried about determining which skills he succeeded in using, and marking them off to see if he can improve them at the end of the session. Second, it means that everyone is on a fairly equal footing, having roughly equal chances of succeeding at any given skill test if there are no skill points attributed to it, and that anyone can make an attempt at any skill and possibly succeed! Third, it means that no matter how far you progress, battles do not get longer, nor do enemies scale up by any great degree. Even though a farmer is nowhere near as scary as a Deep One, the farmer can kill you just as easily with his shovel in the right circumstances.
Well, sure, fine, that is all well and good, but what does that mean for the Game Master? The most important change from a d20 game to a game like Call of Cthulu is one of attitude. Without the need for experience points and leveling up, players feel like their characters are constantly in motion and evolving so long as they participate in making skill checks. In addition, all of the creatures are fairly well balanced, and the only differences between 'weak' and 'powerful' creatures is the tactics that the players will need to use to combat them. Lastly, the players know the fragility of both body and mind in the game, and are less likely to complain at the horrors facing their party: A good game of Call of Cthulu usually has bodies and crazies littering the landscape and you are lucky if none of them are yours.
OK, so the mindset and ruleset of Call of Cthulu are very different from Chaosium to d20 and back again, but really, why should you bother investing in a new system? Unlike d20, which has become an arms race of "Who got the best powerups from the most recent books" (yes, I'm looking at you D&D 3rd and 4th editions), the only person who ever needs to buy more than the main rulebook is the GM. That's right, your shelves need not be cluttered with copious volumes of supplements unless you absolutely want them to be since they have no effect on character creation, skill use, or anything else for the players. Most of the fantastic supplements produced by Chaosium are in the vein of gazetteers, adventures, and creature books, and the only things that the players will get out of them is a knowledge of the background in the Call of Cthulu universe.
As gaming systems go, Chaosium Call of Cthulu 6th edition is definitely a keeper, ranking highly in my esteem and showing us why reading books and looking in dark rooms is scary again!