Gephi is an open source graph visulaization platform that is available free online. There is a great deal of support available on the web in terms of tutorials, videos, etc. Gephi allows users to do a range of network analysis calculations, and users can start with .net and other network file formats as well as .csv spreadsheets.
NodeXL is a free add-on to MS Excel presented by the Social Media Research Foundation.
One new kid on the SNA bloc is Social Network Visualizer, a free program which aims to be as straightforward and user-friendly as possible.
UCI Net/ Netdraw
Text to Pajek