JSONedit

JSONedit is a simple JSON editor with text, tree and list view. Complete application (single executable file) weights about 2 MB (statically linked Lua and Scintilla included), does not require installation and does not depend on any non-standard dll. Settings are stored in .ini file created in application directory, so you can call it portable.
JSON editor

Limitations: no Unicode support. Keep in mind that UTF-8 text with characters limited to 0x00 to 0x7F codes (7-bit) is equivalent to ASCII. Also, if you're using additional characters from single codepage only (your Windows default), then external to/from Unicode converter might come to help. Note: UTF-8 support is present in test version 0.10.

With some limitations program runs under Linux + WINE. Tested JSONedit 0.8 under WINE 1.3 cannot edit large files (very slow operations comparing to Windows) and has problems with immediate tree redrawing when i.e. cloning node multiple times. WINE 1.6.2 seems to work slightly better
JSON editor running under WINE

License: freeware (free as in beer).

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Further ideas & TODO
HERE BE DRAGONS (UTF-8 branch)