Cimolodonts were mammals which existed from the Cretaceous to the Eocene. They were the more derived members of an extinct order called Multituberculata. They probably lived something of a rodent-like existence, until their ecological niche was assumed by true rodents. More basal multituberculates are found in a different suborder, "Plagiaulacida".
In a bit more detail:
Cimolodonta is apparently a natural (monophyletic) suborder. Remains have been identified from across the northern hemisphere. Recognized are the informal Paracimexomys group and the superfamilies Djadochtatherioidea, Taeniolabidoidea, Ptilodontoidea.
Additionally, (and of uncertain affinities), are the families Cimolomyidae, Boffiidae, Eucosmodontidae, Kogaionidae, Microcosmodontidae and the two genera Uzbekbaatar and Viridomys. More precise placement of these types awaits further discoveries and analysis.
Page reference: Kielan-Jaworowska Z & Hurum JH (2001), Phylogeny and Systematics of multituberculate mammals. Paleontology 44, p.389-429.
(This information has been derived from  Multituberculata Cope, 1884. As that's my webpage, there are no issues of copyright Trevor Dykes.)