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.
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 youre 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.