The IBM PC
The first IBM PC was powered by an Intel 8088 microprocessor, was the size of a portable typewriter and packed 16K of RAM. It cost $1,565. It might look horribly dated now, but if it weren't for this first PC we might not have computers at all.
The PC brought computing to the desktop, and its influence lives on. When IBM stopped fighting clone manufacturers and licensed technology to them instead, it led directly to today's modular, upgradeable and customisable machines. When you're upgrading your ageing graphics card to play Crysis or swapping out your old DVD drive for a Blu-ray/HD DVD combo unit, you've got IBM to thank. Or curse.