Replicators are the enemy of the Asgard, more dangerous than the Goa'uld. They appear as mechanical spiders, are intelligent, and ingest metals to replicate. They were defeated on Earth, but continue to plague the Asgard galaxy.
Recently, with the help of SG-1, the Asgard managed to seemingly eliminate the Replicator threat. However, a rogue group were found by Rogue Squadron on P43-3J23.