You might consider mysql.
There's a mysql administrator front-end for OS X as well as Windows that would allow users to add and modify records.....
Downside is the access to administrative tasks right there in front of them and an interface that may appear daunting to everyday users.....
I use MySQL/Apache/PHP combo to throw out the data in a browser to the users and let them have access to only what we want to......they can modify, add, delete records, view reports, etc....but we don't let them modify any table structure or anything of that nature....
Just a thought.
Take care,
C.