Many well-known huge architecture (Facebook, Livejournal, Youtube...) are using it as memory caching to reduce the load on their servers. It can also be used to share PHP sessions among several servers. Let's see how...
The first thing is to install the memcached server on your Debian server :
apt-get install memcached
Then, since the memcache PECL extension now provides its own session handler, it's easy to plug PHP and memcached servers. Just install the appropriate extension (from Dotdeb) :
apt-get install php5-memcache
and change some of your PHP settings :
session.save_handler = files
; session.save_path = "N;/path"
session.save_handler = memcache
; change server:port to fit your needs...
That's all! After relaunching your Apache2 server, your PHP sessions will be stored on the memcached server.