Arrowflight Second Edition is a good example of starting with the campaign setting and designing the system around it. Systems like this are great for small parties who do not want to delve into the glut of the big system, but instead want something of their own.
This is Deep7s second go around with the Arrowflight game system and it certainly shows a good amount of polish and experience. The highly descriptive narrative input in every chapter of the 207-page PDF makes up for the traditional fantasy tropes. But, what sets Arrowflight apart from a good many of the systems out there is its emphasis on the unique fantasy subgenre of magic science. Magic is something that is researched and built upon in this world and it shows. The magic system is robust and one of the most detailed I have seen. If you have been disappointed in how little influence you have on your spell selection, spell potency and spell pizzazz, you will strongly enough playing around with the spells and arcane theories in this PDF.
Character creation is fairly steamlined but quite expansive. You can make a character is as little as 45 minutes or as much as two hours depending on how much of the story your want to put behind your abilities. If you have played a d6 system before, adapting to your gaming group will come easy.
For the Dungeonmaster
Outside of the cool twists on magic, the GM section will really pop out at you. When dealing with an Indie game system, you will not find a lot of ready made adventures. The section walks you through creating your own and even has a nifty randomization chart to spin tales on the fly.
The Iron Word
Arrowflight presents a fun magic heavy campaign world. The theme is integrated well with the system, creating something that plays fluid. It does get long winded in places, but its nice to see passion in a product.