I store my photos on a mirrored RAID array in an HP ProLiant Microserver. So if a drive fails I have them on the other. I wouldn't consider this a "backup" but definitely worth mentioning as this can quickly get me back to my photos.
Also cloud backup at www.livedrive.com via the reseller "Fairly Marvellous", www.fmbackup.co.uk.
I looked at several online backup providers but chose this one because it backs up using Volume Shadow Copy so it's done in real-time. They also do versioning so if I accidentally delete a photo or a folder of photos (or accidentally run a Photoshop batch job against the wrong folder) the original photos are retained. The other killer feature was unlimited backup, I store a couple terabytes with them and never had any complaints from them.