Although it's not content in exactly the same way as a wiki or blog, I have two Del.icio.us accounts with thousands of bookmarks in them, so I also back those up once a month. Again, I don't expect anything to have to Del.icio.us in the foreseeable future, but I've invested a lot of time and effort in collecting and tagging online resources, and it's worth a couple of minutes to me each month to back that up to my hard drive!
To do a backup in Delicious, log in to your account, and then go to Settings, and then choose Backup/Export. You will then be given the option to download an HTML file containing all your bookmarks - just click the Export button. The backup file will not reflect your username so if you are backing up multiple Delicious accounts (as I am), you will probably want to rename each backup file to reflect which Delicious account it contains!
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