The trailer for the new version that’s expected to come this year
http://www.atmosphir.com/
Website for the older version
http://atmosphir.com/index2.html
This is a freeware game where you can make highly customized levels, it’s like Roblox without the lego look and somewhat more versatile (particulary the new version), it also has similarities to the open source Cube/Cube2 FPS game except it focuses more on platforming, is kid friendly, and has a full GUI in which you don’t need any artistic skill or programming skills.
NOTE: If you are squeamish of kids online, better get the old version by registering but not commenting on the forums or news items because kids make up the vast majority of users, but would be quite a boon if you know any kids who like these sort of things.