Troika is free and awesome. Why are you reading this? Just get it!
OK FINE. The osr fighting and fantasy-based system is easy to understand, only requires d6s, uses skills and 3 stats. There is a base Skill number, your natural affinity for doing things without training, Stamina, which is basically HP, and Luck, which can be used as a sort of Save Against X and for other purposes. Skills range from Running to Jousting to Gambling to Axe Fighting, Etiquette and Trapping. In general the skills are descriptive enough you know exactly how they should be used without looking things up.
So Troika can be used to run the same kinds of games you'd run with OSR DnD. So why use it over DnD? Well, I'd say it's even easier to grok than B/X for a newcomer, and quite easy for an experience GM or player to adapt to. NPC stats are easy to improv up for GMs who, like me, don't want to have to carefully create every idiot who will waylay, oppose, or aid the PCs. The system might not supplant all other OSR systems, but that's fine - it's easy enough to learn that it won't be a burden.
But the genius isn't just the system, it's what else you get: the implied setting that you can see through PC backgrounds, the spell list, gear, and NPCs.
The setting is outstanding. There is a mix of science fiction and fantasy, though the world is more fantastical and magical than science fictiony. It's not like Numenera, where all 'magic' is really science. You have demon hunters and skeptical lammasu and androids (all PC backgrounds). Wizards of various stripe, priests, dwarves, gremlin hunters, and lost invaders from other spheres. It is evocative and unique. It is a world I want to explore.
The writing is also quite funny. The Poorly Made Dwarf pc background, the flavor text for the Troll npc, many of the spells, the Tower Wizard... Sell's sense of humor is sprinkled throughout. He hasn't written a parody of OSR settings or the like; I think that what he has is perfectly playable and can portray Serious Important Things. But I can't wait to see a wizard cast Banish Spirit by explaining 'clearly, sternly, why it is impossible that the spirit could be here at this time.'
So, a cool implicit setting, good writing, and a simple system, which means you're not going to spend a lot of time memorizing or learning odd or obscure rules for various things. It's free. Get it.