The level editor source code from Jellify game made by Simon Marynissen and Treazer, which can be played at bonuslevel.org. The game source code and extra resources can also be found on Github.
The actual editor is built using FlashDevelop 5.1.1.1 for .NET 3.5 (master#16708dca57) together with Flex SDK 4.6.0 to target Flash Player 21.0