If you want to host (or have access to) network folders that can be accessed with Windows in Linux systems you can use Samba[1]. You just have to install and configure it which can be very tricky.
On the other hand if you want to have access between Linux computers you can use OpenSSH which is already included in most of the Linux distributions.
[1] http://en.wikipedia.org/wiki/Samba_%28software%29
[2] http://www.openssh.com/
The best free, secure, reliable, and easy solution was the following:
Write a shell script to create backups, call it by cron on a daily, weekly, whatever basis and send the backups to GMail.
Needed items: on the shell gzip and mime-construct (to send emails with attachments), cron jobs, and GMail (or whatever provider with loads of space - GMails current 7GB will be more than enough for the near future).
Shell script
#!/bin/sh
filename1="/tmp/backup_`date +%d%m%Y`.sql";
filename2="/home/www/data.conf";
filename3="/home/www/data-directory/";
target="/tmp/backup_foobar_`date +%d%m%Y`.tar"
myuser="foo";
mypassword="bar";
myemail="foobar@gmail.com";
mysubject="Backup of foobar, `date`";
mydescription="Daily backup of foobar, `date`";
mysqldump -u$myuser -p$mypassword db1 > $filename1;
mysqldump -u$myuser -p$mypassword db2 >> $filename1;
mysqldump -u$myuser -p$mypassword db3 >> $filename1;
mysqldump -u$myuser -p$mypassword db4 >> $filename1;
tar --create --file=$target $filename1 $filename2 $filename3;
gzip $target ;
mime-construct \
--to "$myemail" \
--subject "$mysubject" \
--string "$mydescription" \
--type text/plain \
--file-attach $target.gz;
rm $filename1;
rm $target.gz
If you need additional security, you can of course encrypt the tar file, so your data will be secure, even if someone breaks into your GMail account or you don't trust Google.
If you want you can download a copy via POP3, so you will have backups in your GMail inbox and locally - which should be sufficient for every situation.