Gnumeric is indeed a spreadsheet. It is indeed developed by Ximian (formerly known as Helix Code). It is part of the Ximian GNOME (Formerly Helix GNOME) desktop. Evolution is another part of the desktop, and is a "groupware" (email, calendar, address book, etc) program. And, like all GNOME applications, it's not just for Linux-- it works with all unix apps.
And, despite what people who post things like lists of "why not to use gnome", GNOME does indeed work, is easy to install, and is quite stable. It's also a better development platform than KDE for 2 reasons: easier to access from multiple languages, and it uses the LGPL for libraries, so it's cheaper to use for commercial development.