I'm a huge fan of Alternity. Its core mechanics are a lot closer to what d20 should have been. I encourage everyone to give this system a try and check out its two primary fansites:
www.alternityrpg.net and
www.tequilastarrise.net
Folks have already summarized the system pretty well. Some of the advantages may be less than obvious from the summaries though.
As Derek said, Alternity is strictly skill-based. Every questionable action, including swinging a club or shooting a gun at somebody, is a skill check. This is one of the beauties of the system. It was designed to be a generic science fiction game -- good for any modern or futuristic genre. Because these core mechanics are based purely on character skills, it is amazingly flexible. You can develop any type of character you want in any setting. The classes are extremely loosely structured. Alternity's skill-based combat system just makes far more sense than d20's, particularly with firearms and futuristic weapons.
Alternity's dice mechanics confused some people, but are actually quite simple and very elegant. You roll two dice (a d20 and one other). Roll lower than your skill score and you succeed. The lower you roll, the better your success is likely to be. Your circumstances determine what the second die is and whether its added or subtracted to the d20 to produce your result. (Remember, lower is better.) A super easy skill check could require you to
subtract a d12 from your d20. An insanely hard skill check could require you to
add a second d20. A scale of other possibilities fills in the spectrum. (-d8, -d6, -d4, no second die, +d4, +d6, +d8, +d12, +d20)
The rules allow the GM to just pick the second die based on his "eyeballing" the situation, or he can diligently apply all the relevant modifiers, each one of which moves the choice up or down the scale of difficulty.
In lieu of homogenous hit points, Alternity uses a multi-faceted "durability" system that reflects fatigue, stuns, and a couple types of wounds. As you lose points in each area, your ability to perform is affected. This dramatically changes the strategy of the game, as you're forced to deal with real world consequences of your actions. Durability points don't directly increase with experience as they do in D&D. This too changes the playing field with experienced characters. You rely more on your developed skills to protect you. It makes for much more exciting games.
These factors also make it an outstanding system for espionage games.
Although Alternity was designed for science fiction, many people have enthusiastically converted fantasy campaigns to Alternity mechanics. Derek posted links to some excellent work done on fantasy and magic for Alternity.
Because it was a SF system, Alternity's team spent a lot of energy developing a plausible future history of earth and beyond, and paths of future technology. Elements of this popped up in d20 Future, but the core ideas were woefully glossed over. Without ploughing into the details, I'll just say that Alternity's starship systems are the most fun to play and use of any SF game ever marketed.
The Star Drive setting is a space opera set 500 years in the future. (Several others have already sung its praises.) Those who don't like it, usually call it "too generic", but its breadth and depth and thoughtful execution make it the most plausible future history I've seen in gaming. It avoids hanging on one gimmick or hook like most other settings. Those features have been expanded a hundredfold by fans at
www.alternityrpg.net and
www.tequilastarrise.net
Alternity has its flaws, but the biggest is that it wasn't allowed to mature and develop. Its core systems are so much stronger than d20's, that -- had it not been canceled, but given more professional development -- it would have lead to MUCH higher participation in modern and futuristic roleplaying than today's d20-related offerings have been able to muster.
Carl
One more time:
www.alternityrpg.net
www.tequilastarrise.net
