A wiki is a collection of web pages than is built collaboratively. With most wikis anyone who has signed up for an account with that particular wiki can log in and edit pages. While wikis often have a lot of misinformation published on them, they can still be good starting points for research. Wikipedia is one of the most popular wikis on the web. It contains information about thousands of different topics and doesn't really focus on any one area. There are, however, many wikis that focus on just one subject (see below). From Star Wars to RPGs wikis rock!
YouTube. Pretty much everyone knows what you're talking about when you mention YouTube. But does everyone really know how interactive it is? Every day thousands of videos are uploaded and shared via YouTube. People post their original music there, some people make short films, a lot of people simply make Video logs (it's called a vlog, yo!) about their daily lives. YouTube is a great way to interact with other members of an online community. Almost everyone and their mother has a YouTube account these days.
At first glance Google Latitude seems like a really neat idea. However, when you really think about it, it makes you wonder, "is this really needed?" Google Latitude allows you to see where your friends are on a map that can be displayed on your cell phone or computer. It will only work if your friend gives you permission to do this. While I think that in some cases, such as parents wanting to keep tabs on their teenage children, this idea isn't such a bad one. I have to say though, it does seem a little stalker like. If you really want to know where someone is, why not just call them? If Google Latitude gains enough popularity there is going to be a whole generation of kids growing up using this technology. I don't really know what will happen with Google Latitude, but I'm very interested in seeing what happens with it. Only time will tell.