The basic strategy for HA clusters is to take existing hardware and raise it to the service level of a mainframe, with availability assurances. And while a number of companies offer proprietary HA technology, it has become a commodity drawing interest from the open-source community.
Such IT giants as HP, IBM and Oracle offer their own HA products, along with several software providers, notably SteelEye, Veritas and PolyServe. Open-source providers include Red Hat and FailSafe, with IBM funding research into a heartbeat model for Linux.
HA cluster customers generally are those with enterprise-grade system experience who are familiar with commodity hardware and want scalability, as well as some who are new to the concept but have a specific operational need--such as e-commerce vendors focused on customer service or ISPs boosting their Web-services offerings.