Mayavi!

ParaView is an open-source, multi-platform application designed to visualize data sets of size varying from small to very large.

VisIt is an Open Source, interactive, scalable, visualization, animation and analysis tool. From Unix, Windows or Mac workstations, users can interactively visualize and analyze data ranging in scale from small (<10^1 core) desktop-sized projects to large (>10^5 core) leadership-class computing facility simulation campaigns.

OpenDX gives you new control over your data...and new insights into their meaning. Yet OpenDX is easy to use because it lets you visualize data in ways you've never dreamed of--without getting bogged down in the technology.