Pajek (Slovene word for Spider) is a program, for Windows, for analysis and visualization of large networks. It is freely available, for noncommercial use, at its download page. See also a reference manual for Pajek (in PDF). The development of Pajek is traced in its History. See also an overview of Pajek's background and development.
A detailed introduction to Pajek is given in the book W. de Nooy, A. Mrvar, V. Batagelj: Exploratory Social Network Analysis with Pajek, CUP, 2005. Appendix 4. ESNA in Japanese, 2010. Second edition, 2011. ESNA2 in Chinese, 2012.
For a short description see Batagelj V., Mrvar A.: Pajek - Analysis and Visualization of Large Networks. in Jünger, M., Mutzel, P., (Eds.) Graph Drawing Software. Springer, Berlin 2003. p. 77-103.
An improved version of the paper Batagelj V., Mrvar A.: Pajek - Program for Large Network Analysis, presented at Sunbelt'97, was published in Connections 21(1998)2, 47-57 - (preprint Prison Kinemage).