The Marathon Rubicon team has just posted a series of single-player based maps, the Solo Carnage Pack. The new pack includes over 50 new maps that are designed for solo, but can be used for either solo or network play. According to the Rubicon team:
Featuring 51 original maps, the Rubicon Network / Solo Carnage Pack is a compilation of maps produced during the gameis development for pure entertainment value. No story, no chapter screens, no limits on ammo- the Pack gives you your carnage straight. Think youire tough enough?
You can find more information at the Marathon Rubicon Web site.