Wikis seem to be really, really useful. Having only used
Wikipedia.org myself, I was previously ignorant of all the other Wiki options. I had encountered other Wiki websites (like
Wiktionary), but I didn't realize that I could make my own Wiki. The videos were very easy to follow and helpful - I especially like the Wikis in Plain English video (from commoncraft).
Both Wikis and social networking sites (such as delicious) are extremely effective and concise ways of distributing information. I can personally think of several ways in which I could use Wikis at my current job (where I work with many volunteers and often encounter long, complicated chains of emails). I think one stumbling block for using Wikis efficiently is the problem of getting people into the idea of using them. It can be hard to convince people that using a Wiki would really be that much more efficient than email, but I truly believe that it can be. The other question is getting them to check the Wiki regularly - you could send an email reminder, but then that also raises the question "Well, why bother with Wikis if you have to send an email anyway?"
That being said, with groups where each member will be an active participant (or at least view it regularly) in the creation and modification of the Wiki, I think they could be really useful and save time. I could see a use for Wikis in the classroom - for helping distribute information to and from students and parents. Also, students could create their own Wikis when working on group projects (or the project itself could be to create a Wiki) - this might help solve the problem of one student getting stuck doing all the work because you can easily tell who has altered the Wiki and in what way.
This is technology I definitely see myself using. Delicious is neat, but I'm not sure how much I will use it... I may be proven entirely wrong there, but I'm just not certain.