The Data Den's links page is home to all of the addresses you might need to access the software referenced to create the Data Viz's that lurk throughout the den.
Microsoft Excel: The spreadsheet that everyone's seen. Has some built in DataViz tools and can also be good for handling and exchanging data in a common format with non-techie stakeholders.
NodeXL: Free social network analysis software that is an extension for Microsoft Excel. Can be used for a broad range of social network analysis, but has special functionality for importing and analyzing data from social media sources.
Tableau Public: Tell stories with (quantitative) data. You can use this free version if you can share your raw data and want your DatViz to be fully public. See also Tableau.
Tableau Software: Tell stories with (quantitative) data. You'll need a paid version if you don't want to share your raw data. See all Tableau Public.
Tagxedo: Make word clouds! For free. Great for program covers and PowerPoint slides.