Are you talking about a software application or the actual game collection? I sort of assume it's the former, since the latter would just be whatever you can get in PGN, right?
I use ExaChess - and although it's slightly in need of an update for OSX Lion - I've always liked it better than ChessBase. My needs are perhaps not as sophisticated as some. :)
Are there any Chessbase quality databases for Macintosh?