Web-based GIS/mapping
Most of us are familiar with mapping tools from major companies such as Google (which has a number of third-party front ends, such as Map A List, an add-on that adds info to a Google Map from a spreadsheet). There's also Bing Maps with an API. But there are other options from smaller organizations or lone open-source enthusiasts that were designed from the ground up to map geographic data as well.
OpenLayers
What it does: OpenLayers is a JavaScript library for displaying map information. It's aimed at providing functionality similar to those big companies' code libraries -- but with open-source code. OpenLayers works with OpenStreetMap and other maps.
Other projects build on it to add functionality or ease of use, such as GeoExt, which adds more GIS capabilities. For users who are comfortable hand-coding JavaScript and prefer not to use a commercial platform such as Google or Bing, this can be a compelling option.
Drawbacks: OpenLayers is not as easy to use as, say, Google Maps.
Skill level: Expert
Runs on: Any web browser
Learn more: Try this OpenLayers Quick Start.