Geocommons is a website for uploading and visualizing datasets with a geospatial component (so they can be plotted on a map). Focus is on visualization rather than the data with tagline: "Explore, Create and Share Intelligent Maps and Geographic Data"
License: all datasets licensed under cc by-sa 3.0
Access: data provided in kml or kmz format but difficult to get hold of. Unfortunately a little complex to get data for a given dataset and hard (impossible) to access all datasets at once via web (perhaps can be done via their web api -- requires sign-up and an API key). For example long list of items on doesn't seem to provide link (and the js fancyness doesn't seem to work that well). Also not clear that they make available any of their 'own' datasets