<p>In Eclipse 3.3.2, I would like to replace a character (say ',') by a new line in a file.
What should I write in the "Replace with" box in order to do so ?</p>

<p>EDIT : Many answers seems to be for Eclipse 3.4. Is there a solution for Eclipse 3.3.X ?</p>

Check box 'Regular Expressions' and use '\R' in the 'Replace with' box
It's a new feature introduced with Eclipse 3.4, See
What's New in 3.4