PEBL (Psychology Experiment Building Language) is a system designed for creating psychology and neuroscience experiments and tests. It is cross-platform, with the intention to run the same experiment, unchanged, on Linux, Windows, and Macintosh PCs.
RoboJournal is a cross-platform journal/diary tool written in Qt/C++. Right now, RoboJournal only supports MySQL but support for SQLite (and possibly Postgres) will be added in future releases. RoboJournal runs on Windows and Linux.
I would like to eventually support RoboJournal on OSX but unfortunately I have no reliable way to build or maintain a Mac version right now.
Familiar is cross-platform Family Tree management (Genealogy) Software using QT. It supports viewing/editing file tree using drag-drop in multiple ways. Events allow you to define events in any number.
Prognatus is a cross-platform genealogy project. But it is more than just genealogy. Prognatus will provide routines for maintaining general historic information, searchable thru different view angles, and more!