Try GleedGleed. From its site:
GLEED2D (Generic LEvel EDitor 2D) is a general purpose, non tile-based Level Editor for 2D games of any genre that allows arbitrary placement of textures and other items in 2D space. Levels are saved in XML format. Custom Properties can be added to the items in order to represent game-specific data/events/associations between items etc. Check the video tutorials under the Documentation tab to get an idea of its powerful features.