ChessDB is a free open-source chess database for Windows, Linux, Mac OSX and most UNIX and UNIX-like operating systems. ChessDB is based on code from Scid http://scid.sourceforge.net/ - a project for which there have been no updates for several years. In
It's a solid chess database, for sure. Nothing else free comes close. It has a well-written tutorial, a solid feature set (lost a point for not being able to print games). I will never be a fan of multi-window UI (for the principal reason that, on windows, trying to switch to the application between tasks usually results in only one window gaining focus, with the rest staying hidden). But if you like that sort of design, this is well done, and does everything it is supposed to. It's stable, well-featured (if a bit disorganized), and reasonably easy to work with.