This review is a bit of a departure from the astronomy theme, but the site is such a good idea and I'm sure will be useful.
Wikipedia is an online encyclopedia. Nothing new there, but the difference that Wikipedia is 'open source'. Put simply that means that anyone can create an article, and anyone else can alter or add to existing articles created by others. That might sound like a recipe for chaos, but it actually works very well. For someone to create an article they must be interested in, and presumably knowledgeable about a subject. Others interested in that subject will look at the article and correct any errors and omissions. Rules are in place to prevent the use of articles as advertising and unsuitable content..
The whole thing is very much 'work in progress', having said that there are around 900,000 articles already. Topics in the news get particular focus, and because they can be easily updated online, are always pretty much up to date. This includes space missions like Cassini-Huygens for example.
The home page shows a variety of ways of looking at the information, including a topic of the day, articles related to the current date (this day in history type things) and topics curently in the news. You can even follow a link to a random article.
The quality of articles does vary. The best articles are illustrated with public domain (copyright free) pictures and illustrations and include a number of links to other articles in wikipedia and other relevant websites. The site layout is very clear, and pages print very well for off-line reference.
Back to the astronomy and space theme. You will find loads of relevant and up to date articles on astronomy and space exploration, as this is an area which is well supported on the web. As well as good information the site provides a useful starting point for further links. As such it's one of the first places I visit when researching any subject. Give it an go, and if you're feeling brave create and article yourself.