Slate
A fast, simple text editor that opens files of any size
...The everyday editing is all there: tabs that come back with your unsaved changes, find and replace with regular expressions, syntax colors for 52 languages, line tools, bookmarks and printing. For data files it adds JSON and XML formatting and checking with a structure view, and "show only matching lines" turns a huge log into just its errors.
On Windows, Slate is one signed, portable Slate.exe that updates itself. On Linux it installs from a .deb with its own apt repository. Free and open source under the MIT license; the source is at github.com/jamesccupps/Slate.