Cheapest way to backup data

[100ji]

Hi,
What is the cheapest and safest way to backup data (pictures, documents
etc..) ?

1) having your own raid.
2) Store it on the cloud.
3) Pool free storage (gmail, free online storage etc..)

I have about 100gig of data and I want to store it for the long term. Most
often I noticed that I wouldn't need to access backups, so I feel ok saving
files on gmail.
Is backing up data on the cloud a good idea?

Is there a software that I can use to backup data to these places
automatically?

Thanks,
-S-