What redundant, on-site, low-downtime backup strategies exist for servers?
CrashPlan and TimeMachine have been very helpful for almost all the situations I've seen where one copy isn't enough.

Crash Plan software is very good - watch your java CPU usage until you're comfortable things are running smoothly in incremental mode, but it's as good as any of the other software that's out these days. The software will run on both the sending and the receiving computer elsewhere on your campus/building.

You can of course back up to your offsite storage but once you've got it to the other computer, tapes, off site drive cloning and other non-bandwidth consuming options can work well.

Just watch Java CPU usage when you have large file changes - you can throttle things if needed, but it's pretty self-sufficient with the out of the box settings.