ftp.heanet.ie serves a huge volume of mostly static file downloads with Apache on a single machine. This results in interesting Apache tuning issues, among other things.

• Scaling Apache 2.x beyond 20,000 concurrent downloads [PDF]; just the title alone gives you the flavour.
• Niagara vs ftp.heanet.ie Showdown (Niagara is the new Sun server box and chipset.)
• Niagara Benchmarks: Update

If this sort of thing is your cup of tea, much of Colm MacCárthaigh's blog may be interesting.

(From Tim Bray.)