An American company originally called Helix Code, founded by MiguelDeIcaza (the original architect/author of most of GNOME) and Nat Friedman. The name is a play on "Simian", and they have a strong monkey theme in their branding.
Ximian spent a lot of time making GNOME look pretty, by working on good themes, and also cleaning up applications to make them workable and useful. They wrote applications they believed should exist that weren't around, eg the Evolution groupware/MailClient and the RedCarpet? channel-based package distribution system. They were also responsible for a Free commercial version of GNOME called XimianDesktop, and Mono.
They were purchased by Novell in August 2003, apparently in order to have a decent !GroupWise client for Linux, and for which a connector for Evolution is now in heavy development.
Use this large unofficial Ximian Forum to browse or search the Ximian projects' discussions. The forum is hosted by Nabble which archives Ximian projects' mailing lists into a centralized forum. You can post questions, search for answers, or browse and survey what the community is talking about. You can also drill into the child project forums such as Mono, Gaim, Evolution.