Gephi: Another data visualization tool, Gephi offers a downloadable application focused on complex graphs of network data.
"Raw" Data Visualization Suite: This webpage lets you drag or paste in data and quickly play around with several different visualization formats.
Kimono: A user-friendly version of what's typically called a web scraper. This software tool generates an API from a website of your choosing, allowing you to quickly construct data sets from different types of resources.
Open Data Catalogs: Data Portals.org hosts a huge collection of (collections of) data, organized by place. This is a great source for municipal data sets: For example, zooming down onto Boston will lead you to the Metro Boston DataCommon, where you can explore visualizations relating to culture and the arts, among many other categories, and export the raw data from those visualizations or from new ones you create.
New York Public Library Maps: The New York Public Library has a lot of excellent digitized collections, among them a huge set of maps of New York and other areas. In addition to browsing and search interfaces, they provide an awesome tool called Map Warper which lets you overlay historical maps onto a dynamic present-day map. Check out some Highlights.
Postman: Chrome App for API queries
Tableau Software: Desktop and online visualization tools