I use the Palm version of MobileDB from click here and highly recommend it. It's simple to use and is also MS Access compatible. I'm vaguely aware that Sidekick is some sort of Organiser but I don't know whether it would work with it without copy and pasting.
There's a trial version at click here which, I think, allows you to create a small database.
There's also a version that works with Excel but I haven't used it.