A natty GUI editor is very helpful to create level map. I want to use some open-source choices rather than build my own from scratch. I found Tiled Map Editor but it doesn't work for what I want.

Though I'm building HTML5 game, I don't have to use a HTML5 level editor as long as it can output well-formatted map files which my javascript can read.

Edit:

Sorry for the confusion. Tiled does not work for me because to make the player perform a 'tricky' jump, sometimes I want to set the distance between two platforms to, say, 7/3 or 8/3 tiles. But in Tiled I get only 2 or 3. If Tiled can do this, please teach me.