Unity3D is the most popular engine\editor on the market at the moment and even though I have a few beefs with it, it's actually pretty amazing and currently the best of it's kind. Highly pluggable and uses the awesome mono project. Expensive if you want certain features.
Leadworks 3 is a fairly well known engine\editor as well, recently kickstarting their linux port they have cool features such as visual scripting among others that I will be taking for inspiration!
Duality is a free 2D editor that has some nice 2D specific controls and effects which I would like to copy for the SFML plugin portion of the editor. Is built on C#.
CopperCube 4 uses wxWidgets to create a usable advanced interface for creating games and exports to various targets like Flash, WebGL and Android as well as Windows and Mac. Very good inspiration as it's fairly advanced and feature rich the only issue is it has a hefty price tag.
PolyCode is heralded as the 'Unity Killer' by a bunch of people over the internet for good reason. It's interface is fully customised and customisable and built using PolyCode itself. It's generally useable but doesn't officially provide binaries as of yet.
CraftStudio is a cool multi-user game creation. I think it's tagline: 'CraftStudio: the Game to make Games' is an apt description. The real-time multi-user part is what I'll be copying to Toasty.
Game Maker: Studio An amazing piece of software for creating 2D games, IMO it's better than Unity3D (including with the latest 4.3). Is insanely expensive.
NeoAxis is a fairly mature piece of software, has some weird UI choices but interesting to follow and it's examples are awesome. Recently came
out with a free version.
Construct2 A very nice piece of software with lots of different exports available, it's has only UI scripting available but is still pretty powerful. It keeps a very native windows feel (Think the new Office).
Ogitor a not so mature project, however is usable for Ogre projects I used old versions of this to help me get Ogre working on Mac. It's good for any integration with Ogre and is very pluggable.
Tiled is not for making games, but for making the maps for games. It has some advanced tile map functionality which I will also be stealing and adding some of my own additions. I'll be thinking about making the tiled format loadable for my editor to test out my tile map features.
Cool engines that I also watch:
If you know of any other editor \ engine that could be great for inspiration or even using feel free to leave a comment.