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.