OpenStreetMap
What it does: OpenStreetMap is somewhat like the Wikipedia of the mapping world, with various features such as roads and buildings contributed by users worldwide.
What's cool: The main attraction of OpenStreetMap is its community nature, which has led to a number of interesting uses. For example, it is compatible with the Ushahidi mobile platform used to crowdsource information after disasters such as earthquakes. (While Ushahidi can use several different providers for the base map layer, including Google and Yahoo, some project creators feel most comfortable sticking with an open-source option.)
Drawbacks: As with any project accepting public input, there can be issues with contributors' accuracy at times (such as the helicopter landing pad someone once placed in my neighborhood -- it's actually quite a few miles away). Although, to be fair, I've encountered more than one business listing on Google Maps that was out of date, too. In addition, the general look and feel of the maps isn't quite as polished as some commercial alternatives.
Skill level: Advanced beginner to intermediate
Runs on: Any web browser
Learn more: See the Quick Tutorial on the OpenLayers site.
Temporal data analysis
If time is an important component of your data, traditional timeline visualizations may show patterns, but they don't allow for sophisticated analysis or a great deal of interaction. That's where this project comes in.
TimeFlow
What it does: This desktop software is for analyzing data points that involve a time component. In a demo I wrote about before its release, creators Fernanda Viégas and Martin Wattenberg -- the pair behind IBM's pioneering Many Eyes project who later left for Google -- showed how TimeFlow can generate visual timelines from text files, with entries color- and size-coded for easy pattern spotting. It also allows the information to be sorted and filtered, and it gives some statistical summaries of the data.
What's cool: TimeFlow makes it easy to interact with data in various ways, such as switching views or filtering by criteria such as date ranges or earthquakes of magnitude 8 or more. The timeline view offers a slider so you can zero in on a time period. While many applications can plot bar graphs, few also offer calendar views. And unlike web-based Google Fusion Tables, TimeFlow is a desktop application that makes it quick and painless to edit individual entries.
Drawbacks: There are no facilities for publishing or sharing results other than taking a screen snapshot, and the code hasn't been updated in several years.
Skill level: Beginner
Runs on: Desktop systems running Java, including Windows and macOS X
Learn more: Check out Top tips.
Note: If you're looking to publish visualized timelines, better options include Google Fusion Tables, VIDI or the SIMILE Timeline widget.