simple and user friendly multi-platform database (software or) workflow [on hold]

What is a good workflow for dealing with relatively small databases in a simple and robust way in Mac OS X (or Unix), Windows (or dos) and Linux?

I'm not an IT professional. But over the years, I came to appreciate the simple solutions, that are more universal, and though they are not “user-friendly” at a first glance, they are more elegant, powerful, and useful in a long term. Thus I use, for example, pandoc markdown in Vim -- instead of LibreOffice Write, Apple Pages or Microsoft Word.

With that in mind I've been searching for a database workflow for some weeks and still can't find it. What I search is:

1. A simple, lightweight, solution;
2. Cross-platform;
3. Must have a friendly interface to input data (since other colleagues may also input data in the systems that I build);
4. Must be accessible in a long term.
5. Preferably, it may be free.