LAN Mirroring technologies are an important HA technology now, and will remain a key ingredient in many inexpensive HA systems. They allow two machines to mirror a filesystem between themselves with only two NICs and a crossover cable - yielding a system with no SPOFs for a very low price.
DRBD: system by Philipp Reisner and LarsEllenberg for mirroring filesystems across the LAN. It is RAID and LAN aware with a quick sync option for bringing the mirror up to date quickly. Recent versions even have a very fast full-sync option. This is the general replication technology which is most commonly used by members of the Linux-HA community, because it provides a complete solution, is sensitive to data integrity issues, and integrates easily with Linux-HA.
NBD server software - the complement to the in-kernel NBD driver.