What are good open data sources for open data?
data.gov is nice but limited to American open data.
Are there other, similar initiatives?
Check out opendata.arcgis.com/explore.json. It's a good source for open data that is (mostly) spatial. Soon there will be a master search to look through all these sites at opendata.arcgis.com. There will also be a showcase of individual sites you can link to from there.
Full disclosure: I'm a product engineer on the Esri Open Data team.
http://opendata.stackexchange.com fits your requirements since: