The metal spiders were simplistic and subtle robotic servants used by Cassandra O'Brien for infiltration, observation and sabotage.
The metal spiders were small robots with four tentacular appendages and two saucer-shaped body parts. The top part had a red eye which could emit light. Individually, they weren't very strong or dangerous, but could be formidable in large groups. Their small size, great mobility and large numbers made them highly effective as sabotage devices. (TV: The End of the World)
The metal spiders were transported by metallic orbs, which were in turn transported by the Adherents of the Repeated Meme. Cassandra used them to take over the mainframe of Platform One, destroying the systems which protected it from the Earth's destruction to collect compensation so she could use it on her innumberable beauty operations. (TV: The End of the World)
On New Earth, Cassandra used the metal spiders for surveillance both inside and outside the New New York Hospital. She used them to spy on the arrival of the Tenth Doctor and Rose Tyler on New Earth. However, Cassandra's death not long after this observatory mission led to the end of their use. (TV: New Earth)