A distributed miner is a Bitcoin miner made from the pooled efforts of multiple computers, possibly without the computer user's knowledge.
Usually, the theory behind distributed miners consists of dividing work into workunits and sending them to various computers, who all report back with their work to be submitted en masse by a central processor. Alternatively, though, the computers can coordinate themselves to submit without any central puppetmaster.
Distributed Miners in the Wild
Stealthcoin employs a method not unlike this, where unused computers act as bitcoin miners. It currently does not, however, coordinate between bots.
Bitcoin Plus allows the distributing of works amongst friends using their client, or through an embeddable website script