Online Backup For WordPress is a plugin that backs up your site to a remote server at BackupTechnology free of charge including 50 MiB of free space. You will need to register for a free account.
More Info: http://www.backup-technology.com/online-backup-for-wordpress/
WordPress Automatic Online Backup Plugin provides a similar function, with 5 mb space. you need to be able to run sql queries on MySQL servers.
More Info: http://wordpress.org/extend/plugins/wordpressbackup/