Slate
A fast, simple text editor that opens files of any size
...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.