Well, we don't have many clusters that are purely static, but we do have one small one, our image cluster (most of our images are sent through CDNs like Akamai). We put up a Linux 2.4.18/Tux 2.1 server in that cluster... and holy CRAP was it fast.
Through some quickie ab (apache benchmark) benchmarking, I found that it really was fast - damn fast. In fact, a single box running Tux was showing about 10x the throughput of a pair of FreeBSD boxes running Apache, and that's even with the new accept filter support turned on. Hot damn.
So, the image cluster will likely be moved over to Tux in the near future. I'm still playing with it to make sure it doesn't break anything, but it seems to just be blindingly fast, with no real downsides (other than the fact that it doesn't do referrer checking, which isn't currently a problem but could be one in the future, as I have plans to put it in play in a few other places, and that it doesn't do rewrites, which is another large concern, as I need that functionality in some places. Still... wow.).
It's inspired me to the point that I'm thinking of teaching myself a little c so I can write a tux userspace module (even scarier is that I'm thinking of writing a simple version of Apache's mod_rewrite, which should be pure pain). It's definitely not the sort of thing most people would recommend taking on for a first project, so of course that just makes it more likely that I'll try it. ;) Oh well, can't stick to perl forever.