Greg Egan's Orthogonal Trilogy is perhaps the ultimate in hard sci-fi world building: starting by simply "changing a minus sign to a plus sign in a simple equation that governs the geometry of space-time", Mr Egan then derived, from first principles, how light, matter, energy, motion, gravity and time would work in this alternate universe. The result is 80,000 words on his web site describing the physics and cosmology of the universe (minor spoilers) and three novels: The Clockwork Rocket (extract), The Eternal Flame (extract) and The Arrows of Time (extract) [more inside]
92 Gears is a lovely, hypnotic animated GIF. Ball Bearings in a Hypersphere is a mathematical discussion of its generalizations.
StarshipSofa has podcasted all of the Nebula Best Short Story Nominees for 2008, following on from podcasting all but one of the 2008 BSFA short story nominees. Previous StarshipSofa.