VisIt is an open-source, interactive parallel visualization, and graphical analysis tool designed for viewing scientific data. It can visualize scalar and vector fields on 2D and 3D structured and unstructured meshes. 
History.
VisIt was developed by the Department of Energy (DOE) Advanced Simulation and Computing Initiative (ASCI) to visualize and analyze the results of terascale simulations. It was conceived as a framework for adding custom capabilities and rapidly deploying new visualization technologies. After an initial prototype effort, work on VisIt began in the summer of 2000, with the initial version released in the fall of 2002. 
VisIt is supported by the DOE's SciDAC (Scientific Discovery Through Advanced Computing) program to assist with the analysis and visualization of large datasets generated by open scientific computing. It is developed in collaboration with multiple organizations, including Oak Ridge National Laboratory, Lawrence Berkeley National Laboratory, and the University of California, Davis. Being open-source and released under the BSD license, VisIt has received contributions from various other developers.
Summary.
VisIt is an open-source application for large-scale simulated and experimental data sets. The application serves as an infrastructure for parallelized, general post-processing of extremely massive data sets. Its primary use cases include data exploration, comparative analysis, visual debugging, quantitative analysis, and presentation.
VisIt leverages several third party libraries: the Qt widget library for its user interface, the Python programming language for a command line interpreter, and the Visualization ToolKit (VTK) library for its data model and many of its visualization algorithms. The VisIt-specific effort has largely been focused on parallelization for large data sets, user interface, implementing custom data analysis routines, addressing non-standard data models (such as adaptive mesh refinement (AMR) and mixed materials zones), and creating a robust overall product. It has been ported to Windows, Mac, and many Unix variants, including AIX,
IRIX, Solaris, Tru64, and Linux, including ports for SGI's Altix, Cray's XT4, and many commodity clusters.
The basic design is a client–server model, where the server is parallelized. The most common form of the server is as a stand-alone process that reads in data from files. However, an alternate form exists where a simulation code can link in "lib-VisIt" and become itself the server, allowing for in situ visualization and analysis.
VisIt follows a data flow network paradigm where interoperable modules are connected to perform custom analysis. The modules come from VisIt's five primary user interface abstractions. There are twenty one "plots" (ways to render data), forty-two "operators" (ways to manipulate data), eighty-five file format readers, over fifty "queries" (ways to extract quantitative information), and over one hundred "expressions" (ways to create derived quantities). Further, a plugin capability allows for dynamic incorporation of new plot, operator, and database modules. These plugins can be partially code generated, even including automatic generation of Qt and Python user interfaces.
The VisIt project originated at Lawrence Livermore National Laboratory as part of the Advanced Simulation and Computing (ASC) program of the Department of Energy's (DOE) National Nuclear Security Agency, but it has gone on to become a distributed project being developed by several groups. Major hubs for the project come from:
The project has twenty developers from many organizations and universities, including five DOE Laboratories.
References.
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