MyShelf is a universal note taking app for plain text snippets, pictures, links.
Platforms: It is available on all major platforms. For Windows and Linux there is also a "Portable Edition". The pure-Java version for all platforms works also on Mac OS.
Download packages (Column Portable Editions): http://encomit.de/myshelf/desktop.html#download
Dependencies: The Windows-Exe ist Self-Extracting (to arbitrary drive/folder/location for the Portable Edition) and contains an embedded JRE. The rest need Java to be installed.
All data is always stored next to the binaries (Folder "data").
Stealth: Not sure, but I would believe so