High-availability file system
I may be beating a dead [high availability] horse here, but I just wanted to put this out in the open because I don't ever recall seeing before it on LET (for a server-side solution, at least).
Was looking around a bit for replication solutions for the file system (after @netomx asked for HA solutions) and rsync is really the way to go (industry standard type stuff?) even given some of its shortcomings but I did find one sweet alternative.
I came across a project called SparkleShare. I think this is aimed at a more consumer level, but I see absolutely no reason why it can't be used to sync 2+ servers. It uses git as the backend so file syncing should happen immediately (as opposed to a rsync cron) and with less overhead because it doesn't need as much processing power as rsync.
On top of this amazingness, SparkleShare is a two-way street which means while your main server is down, the backup servers have to ability to bring the main server up to date after it comes back online.
Plus, it seems like it's under very active development.