Here you will find the packages with the current simpleSAMLphp versions:
Check the archive in case you are looking for any other older version. Older versions (for archiving purposes):
You may also checkout the latest developer version using subversion:
svn checkout https://simplesamlphp.googlecode.com/svn/trunk/ simplesamlphp
Make sure you follow the instructions to install from subversion.
The core library handling the basic SAML stuff (messages, bindings, and so on) has been detached from simpleSAMLphp, and can now be found in github:
SimpleSAMLphp is available as a Debian package. You can therefore install it with apt-get, but bear in mind that the version available in Debian stable might be outdated.