Free for Windows
A pinned note behaves as part of its window. Drag the window and it follows, minimise and it hides, close the window and open it again and the app recognises it and re-attaches the note. There are three ways to pin one, and which is easiest depends on where your hands already are.
No installer · a single executable · touches nothing in the registry · v2.6.0
Hover a note and icons appear along the top. Click the lock: the screen dims and a preview of the note follows your cursor. Click where you want it and the note moves there and pins. No snapping heuristics get involved, so it lands exactly where you pointed.
Hold Alt and right-click on any window to get a menu for that window, with its name at the top. From there you can attach a new note or a new label to it directly, without taking your hands off the program you were using.
Drag a note near the edge of a window and it snaps to that side; drop it on top of the window and it pins inside. To move an already-pinned note to a different window, hold Ctrl while dragging.
Does it work with windows running as administrator?
Yes. Some windows send no accessibility events, so a polling watcher reads the window rectangle directly as a backstop.
If a note sits over a window, does it block clicks?
Turn on click-through for that note and clicks pass to the window underneath. Combined with fade-when-not-hovered, the note stops covering anything.