iPXE is the leading open source network boot firmware. It provides a full PXE implementation enhanced with additional features such as:
boot from a web server via HTTP
boot from an iSCSI SAN
boot from a Fibre Channel SAN via FCoE
boot from an AoE SAN
boot from a wireless network
boot from a wide-area network
boot from an Infiniband network
control the boot process with a script