Gave ZOME a test drive, and it's pretty slick and easy to use. Omits a lot of the cruft that many editors like this bring. I particularly like the way you accommodate resizing the map - very clever, clean, and intuitive.
It does feel like it needs a few things to be usable in a project, though. Some of them ought to be easy to do, some might be more difficult:
* A "fill" feature would be nice.
* Like you said in your page, layers are kind of needed.
* Some way to associate metadata with locations in the map. Things like entrance locations, exits, npc's, etc. The way Tiled handles it is that you can add dictionaries of key/value pairs as "objects" to location, which is really powerful, but if it were just associating numbers with tiles (like sticking a little "12" in the upper left corner of a tile when the number 12 is associated with it), that would be enough for a lot of purposes, since you could map those numbers to external things elsewhere.
* Some way to associate metadata with the map itself would be nice, too.
* The first tileset I imported was my tileset from Arkham After Midnight, which is a 512x512 tileset with 16x16 tiles. It was far too small to work with the tile selector on the right. You might consider letting the user zoom the tileset like you allow them to zoom the map.
* I noticed that the tileset selector area was vertical, so I tried making a quick custom tileset that was 256x512, which was definitely more usable, but the UI cuts off the tileset view at about the 21st row instead of showing all of it, even though there is plenty of room. (I'm guessing you're expecting only square tilesets?) You might consider letting that entire right pane be filled with the tileset.
* I'd like to see menu commands for things like save and load instead of just keyboard commands.
* Undo/redo support would be nice.
Anyway, a nice editor. I could see using it for quick one-offs, or games with simple tile needs. I'd love to see it get developed a little further to add metadata and other amenities; I'd love to be able to use something like this for Arkham After Midnight and similar projects.