Our favorite here at STUG is Carbonite. Steven, Jared, and Travis all use it. Carbonite is a subscription service at $50/yr. If free is more your thing, Mozy is a good choice. Mozy provides 2Gb of backup for free. This would be good for your notes but not much else. Jungle disk is a cheap pay as you use service. With this service you have unlimited storage and no set monthly/early fee. However, it does cost 15¢ per gigabyte and I believe there are monthly transfer limits. This service is provided by Amazon using the Amazon S3 service.
There are other options, Microsoft offers an online data storage service. This option may be more atractive once Microsoft Mesh is released to the public.
The best advice is to research all of the options and decide which fits your needs. Obviously, if you need to back-up large quantities of data a service like Mozy won't work without paying the monthly fee. Another fact to keep in mind is compatibility, I believe you are using Windows, however, if not remember that Carbonite is not Unix compatible, while Jungle Disk is cross platform. Also, Jungle Disk provides the ability to access files from other computers while Carbonite and Mozy are more for backing up files and disaster recovery.
There is an entire episode of Tablet Tips dedicated entirely to this subject: Getting Organized: Online Backup