Crash Plan

CrashPlan offers lots of backup features and the price is great - it starts free.

With the free plan you can backup to an external drive, netowkr drive, or even another computer over the internet (like a friends computer).

For $3.50/month you can backup unlimited data from a single computer to the crashplan servers and for $5/month you can backup unlimited data from multiple computers to the crashplan servers.

Their software works on windows, linux, and mac.

I am currently using the free plan. I bought a cheap intel atom machine for under $200. It's ultra quiet and low power and I run CentOs on it and it's sitting at home on my verizon fios line. All my office machines then backup to it over the internet. It's a great way to do offset backup extremely cheap.