Ask HN: Do non-commercial offsite backup solutions exist?

Crashplan does exactly as you mention, and handles jumping through NAT, encryption (with your own keys if you want), etc:

http://crashplan.com/

Cross platform (Mac/Win/Linux/Solaris). Some extra features unlock if you buy the + version.

If you want a totally free solution that would take a little more work to implement, look at Duplicity:

http://duplicity.nongnu.org/

rsync based, uses SSH/GnuPG for security.

Run one of these on whatever hardware you have available.

You can easily tunnel rsync over SSH. You can configure even the simplest routers to do port-forwarding, e.g. your friend's external IP on port 2222 is redirected to port 22 on an internal host.

Rsnapshot (rsnapshot.org) is a free rsync-based backup system; it is very efficient in terms of disk space and keeps full backups, but only keeps 1 copy of each file.

I use an old ppc mac mini and openvpn to do remote timemachine backups. It works pretty well and was pretty easy to setup.

you might be able to use local-drive backup software, a drive at your friend's home, and a pogoplug device to access the drive remotely as if it were a local drive.

http://pogoplug.com/

and you could use TrueCrypt to add an additional level of security.