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Most people use Linux virtual server (LVS) which is a highly scalable and highly available server built on a cluster of real servers, with the load balancer running on the Linux. Visit The Linux Virtual Server Project - Linux Server Cluster for Load Balancing site for more information. Can you tells us a bit about your application? Do you want failover or load balancing or combination of both? Do you want SSL support? Do you use centralized SAN storage? What about MySQL/Oracle? Do your need solution for that too...

So many factors more you tell us the right information you will get...