CartoDB has a pretty low learning curve but creates excellent visualisations including ‘Torque’ views which allows you to ‘play’ data along a timeline. To date (May 15) this is the first free tool of its type we have seen. Here is an example which plots tornados across North America:
CartoDB also allows you to build your own later and point data and export it in csv, shp, kml, svg and geojson – excellent if you are building a library of layers to use in many of these and other tools.
Simple email registration is required, though it can be linked to a Google account. You can store up to 50MB of data for your maps on a free account, which is fairly generous.