A killer micro is a microprocessor-based machine that infringes on mini, mainframe, or supercomputer performance turf. Often heard in "No one will survive the attack of the killer micros!", the battle cry of the downsizers. Used especially of RISC architectures.
The popularity of the phrase "attack of the killer micros" is doubtless reinforced by the movie title Attack of the Killer Tomatoes (one of the canonical examples of so-bad-it's-wonderful among hackers). This has even more flavour now that killer micros have gone on the offensive not just individually (in workstations) but in hordes (within massively parallel computers).