5.3 is rather big release including support for namespaces, closures, phar archives, internatioalization support via the new intl extension, improved SQLite support, mysqlnd as backend for the MySQL exensions, impressive performance improvements, ... and tons of other bigger and minor things.

You can grab this latest download over on PHP.net QA site includng both the source for unix-based machines and packages for Windows.

Chris Jones has also posted some thoughts on the new release candidate and, of course, here's the official announcement from PHP.net.