Michael Lee at the Addison Road blog has put together a nice tutorial for doing an online backup to a DreamHost account. DreamHost is a well known shared hosting provider that offers a reasonably good deal on website hosting, with about 170GB of storage for about $8 a month. Michael is basically suggesting that if you have a site with DreamHost, but don't use anywhere near your allotment of storage and bandwidth, you can use some of that space for online backup instead of one of dedicated online backup services like JungleDisk with S3, Omnidrive or Joyent BingoDisk. Michael's solution involves using OS X's built-in command line tool rsync to do the heavy lifting (the actual backing up), scheduled using iCal alarms calling an AppleScript.
In fact, I have been a DreamHost customer for quite some time, and I use my hosting account like this as well. Instead of using rsync like Michael, however, some time ago I set up an Automator workflow that calls the Transmit FTP client to upload backups to a WebDAV folder as suggested on the Strongspace blog. One advantage of doing it this way is that I can mount that WebDAV folder in the Finder to grab files off of it (or even access it through a web browser). However you do it, taking advantage of online storage space you might already be paying for is a good idea.