Memcache is the best caching API for php, but it is not available on most of the hosting providers. Are there any alternatives to memcache which are available for most hosting providers(e.g. Godaddy)?
Several of my web applications and web hosting provider uses Advanced PHP Cache, also known as APC, instead of Memcache. From my experiences of using both, APC has improved performance more.
I would make sure to ask your web hosting provider if they are/can use APC.
For non shared hosting, I would say Varnish will be your best option. It is a reverse proxy and cache content as well. This would result in improved performance of apache, reduce server load and would result in faster page loading times. There is an interesting discussion at Drupal regarding various caching options for it. In fact, most of them apply to any php based system. You may find them useful.