Since I started to use XeTeX, I found out that having many font families installed is very useful when we want to change the look of the document?
Does anybody know a place to get good fonts (if possible, families)?
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If you're looking for free license fonts, try FontSquirrel.
You have to be somewhat careful looking for fonts online, since some of the so-called "free font" pages online are distributing fonts illegally. FontSquirrel at least provides info on the license.
Here is a LaTeX font related site. Not sure if it is related to xetex or not?
SIL International maintain a list of free fonts covered by the Open Font License. As examples, Inconsolata, Linux Libertine, Andika, Gentium and STIX.
Not updated for a few years, the Unicode Font Guide For Free/Libre Open Source Operating Systems still has some useful information.
The Gnu freefont was designed with "desktop publishing" in mind but also, to quote from their own write up, they "are especially pleased with the Mathematical Operators range, with which most of the glyphs used in LaTeX can be displayed."
Finally the TUG fonts page includes many free fonts with details of licenses.