Sublime Text 2 is mostly coded in C++ and uses a custom UI toolkit. Here is the author, Jon Skinner, explaining it: http://news.ycombinator.com/item?id=2822114.
I keep meaning to write a blog post with some details on this, but as
with many things, I usually end up coding instead. Sublime Text 2 is
almost entirely C++ (with a smattering of Objective C for Cocoa and
Python for plugins). Coding is generally fairly straight forward: code
on one platform (mostly Linux at the moment, but I switch around
frequently), and then make sure it still compiles elsewhere. Sublime
Text 2 itself uses a custom UI toolkit. There are a lot of apps where
this may not make sense, but it’s not such an unreasonable choice for
Sublime Text, where I always knew that a lot of the UI controls were
going to have to be custom no matter the toolkit (e.g., the text
control and tab controls). The UI toolkit sits on top of a cross
platform abstraction layer, which is more a union of platform
functionality rather than lowest common denominator.