I’d like to backup my PC, preferably online so that the backups are out of the way of theft, fire or whatever. Can anyone recommend an online storage system, or are there other options?
Mozy.com allows free backups of up to two gigabytes, automatically. (It can back up your files at night, or after a period of inactivity.) Five bucks a month gets you unlimited backups. Note that this price is only for personal (not commercial) use.
I started a similar thread in February, and wound up using MediaMax: 25GB of storage for free, the interface is friendly and intuitive, etc. I’ve been really happy with it. I don’t use it for full computer backups, but it’s certainly set up to handle those. I recommend checking out the replies in my old thread, because there were some good suggestions in there.
Good luck. 
I don’t know what your upstream bandwidth is like, but chances are it’s not great. For example, I have 10 Mb downstream, but only 512Kb upstream. This means that backing up will take a long time. I suggest getting a USB HDD or dual-layer DVDRW, depending on the amount of data, copying off your data, and taking the HDD or DVD to work and leaving it there. Do this on a regular basis.
Thanks folks. Quartz, carrying a backup is a good idea, though I’d really like to upload.
Are you a developer? If so, you could use Amazon.com’s S3 web service. The prices are very reasonable: $0.15 per GB-month for storage, $0.10 per GB for inbound transfer, and $0.18 per GB for outbound transfer. It’s not free, but it’s a great, scalable service.
If you’re not a developer and just want some client programs that use S3, Wikipedia has an article that lists some S3-enabled programs.