This Gizmodo article (How to back up all your stuff for free) has a good list of the major providers of storage on the net. Using this space as a free backup is a great idea, but when you start seeding the clouds with your data, you should consider:
I'm looking for a complete cross-platform solution, open source, with desktop integration and transparent but trustable encryption. Dropbox is the closest at the moment for me, but it's limited to 2GB. It does allow me to keep files in sync simply between Linux and Windows machines, however, which is awesome.