If you haven’t already, go watch Eskil Steenberg’s GDC videos of his game Love, and the tools he uses to build it. The game itself is fresh and beautiful, but as an programmer, it is his tools that have blown me away.
The last game development tool to really catch my fancy was Carmack’s co-operative world and megatexture editor for ID Tech 5, which was demoed back in 2007, but I can honestly say that it pales by comparison to Eskil’s tools.
And the best part? All of Eskil’s tools are open-source.