ESRI's ArcGIS Online is a web-based, user-friendly GIS that includes spatial analysis tools and many layers of prepared data; check out the Gallery for examples. ArcGIS Online can be used with ESRI's Story Maps which are easy-to-use templates for publishing popular types of web maps and apps.
Geocommons "enables everyone to find, use and share geographic data and maps. Easily create rich interactive visualizations to solve problems without any experience using traditional mapping tools."
The New York Public Library's Maps Division developed the Map Warper, an online tool that allows the easy alignment of historical (mostly fire insurance) and digital maps. This alignment allows one to connect historical maps to many other sources of data to further the analysis of the history of New York City and environs.
The University of Virginia has created Visual Eyes which is a "web-based authoring tool" that "weave[s] images, maps, charts, video and data into highly interactive and compelling dynamic visualizations."
Hobart and William Smith Colleges host NEARCHUS, which "provides a simple and effective mapping tool that allows an instructor to determine specific questions or criteria for students to research. The application plots locations on a Google Map and creates a set of fields into which are placed student responses to these questions, including any images."
Ushahidi is an example of a crowd-sourced disaster response system, with examples from Haiti and Kenya.
MapStory "empowers a global community to organize knowledge about the world spatially and temporally. With MapStory, people of all kinds turn into Storytellers who can create, share, and collaborate on MapStories and ultimately improve our understanding of global dynamics, worldwide, over the course of history."
Originally created for journalists, Knightlab's StoryMapsJS is an easy-to-use tool "to help you tell stories on the web that highlight the locations of a series of events." There are a number of examples which give a sense of what kind of narratives are appropriate for this tool.
Social Explorer provides easy access to demographic information about the United States from 1790 to the present.
Another tool for census data exploration via mapping is the Historical Census Browser from UVA Library.
The US 2010 Research Project examines changes in American society in the recent past. Create maps of census data for counties and neighborhoods anywhere in the U.S. and as far back as 1940.
One may also explore census data and demographic trends with Census Scope, brought to you by the Social Science Data Analysis Network (SSDAN) at the University of Michigan.
The American Museum of Natural History is developing Digital Universe, a downloadable 3D tool that enables you to explore the universe.
The free map viewer Celestia "lets you explore our universe in three dimensions."
Another product is SkylineGlobe, a free viewer which is part of a suite of products which enable manipulation of 3D data.
Microsoft's search tool, Bing, includes map apps and the ability to create maps of your own.
Open Street Map is "a free editable map of the whole world. It is made by people like you ... [it] allows you to view, edit and use geographical data in a collaborative way from anywhere on Earth."
Policy Map "is a fully web-based Geographic Information System. It's fast, efficient and captures data in visually powerful ways through custom demographic maps, tables, reports and our analysis tool, Analytics." (fee-based)
Axismaps offers custom mapping services and interactive map hosting (fee-based) plus some free resouces.