"The MySQL database is an Open Source application which boasts millions of installations worldwide and is available free under the GPL from the MySQL AB website. (It is also available under a commercial license, but unless you're planning to use it in a closed-source application, this shouldn't be of concern.) It's a popular choice in particular for use on the backend of websites, especially in combination with the PHP scripting language, but it's also compatible with Perl, Python, C, Ruby, and just about any programming language that supports ODBC. MySQL runs on over a dozen operating platforms, including Linux (of course), Mac OS X, Solaris, and Windows, and its behaviour varies very little between operating systems. In fact, in many cases, you can copy MySQL database files between machines running MySQL on different operating systems, restart MySQL, and have immediate access to the data. (If you've been running MySQL on another platform, this makes it easy to migrate your databases to Linux...)"
