EverNote is sorta cross-platform with the coming web-based version.
Off the top of my head, I only know of two pure cross-platform apps like you ask, KDE Pim (way to go, an original name
), and Evolution, but those are FULL PIMs, maybe too full to replace OneNote, and both are flaky under Windows. OneNote size is bigger though, but in features is slimmer (no mail, RSS reader, etc.). A quick search over the giant thread
only uncovers NoteCase Pro.