From their site: del.icio.us is a social bookmarking website -- the primary use of del.icio.us is to store your bookmarks online, which allows you to access the same bookmarks from any computer and add bookmarks from anywhere, too. On del.icio.us, you can use tags to organize and remember your bookmarks, which is a much more flexible system than folders.
The API provides read/write access to Delicious bookmarks and tags via an HTTP-based interface. The RESTful API returns responses in XML.
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Faves supports multi-word tags, and multi-tag strings are separated by commas. In del.icio.us, tags are space-delimited and must be single words. By default, results from the Faves del.icio.us API are modified to be space delimited (spaces in individual tags are replaced with underscores). Thus, the tag string "harvey danger, digital music" would become "harvey_danger digital_music" (this applies to both /posts and /tags).
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REST is a pretty good standard, easier to implement than SOAP in my opinion. It really depends on your tools, I suppose. I wonder how many people request SOAP when REST is available. I prefer REST+XML, JSON is bonus in my books. I end up using regex for most of my work I must admit.