Below is a really simple interactive map hosted on Gist (as a Gist file named *.geojson). The steps to make this are remarkably simple:

1. Create some geojson (open source geo java script object), for example by just drawing features and letting it right the JSON for you in geojson.io.
2. Save as Gist (login with your  GitHub credentials)
3. You can then share the Gist (like this one Matt made), or embed it in a page on another website.

Geojson.io is a good way to create some GIS data quickly, and to teach yourself geoJSON.