Many salt-loving archaea, or haloarchaea, commonly harbor one or more megaplasmids in their genomes. The haloarchaeal model organism, Halobacterium sp. NRC-1, contains two related replicons, pNRC100 (191 kb in size) and pNRC200 (365 kb), both of which code for buoyant gas vesicles as well as other genes that are important or essential for the host. Two other haloarchaea, Haloarcula marismortui and Haloferax volcanii, with larger genomes have a more complex complement of extrachromosomal replicons, eight in the former, including four megaplasmids, and four in the latter, including two megaplasmids and one very large plasmid. Two other sequenced haloarchaea, Natronomonas pharaonis and Haloquadratum walsbyi, have fewer and smaller extrachromosomal replicons, with a single megaplasmid in the former and only a relatively small plasmid in the latter. We review the current state of knowledge on these sequenced megaplasmids, including the eukaryotic type replication protein genes present in most, IS elements populating many, and unusual (e.g., gas vesicle), important (cytochrome oxidase and thioredoxin/thioredoxin reductase), and even essential (transcription and replication factors and an aminoacyl-tRNA synthetase) genes on several of these large replicons. The role of megaplasmids, some of which may qualify as small chromosomes (minichromosomes), in the evolution of haloarchaeal genome architecture is discussed.