Template:DEFAULTSORT - Place on articles (Traditionally, at the end, before the categories) whose title begins with "The" or is the name of a person (for example). This lets the MediaWiki software know where to place the article in alphabetical lists
Template:Fact - Add at the end of statements which you believe should have an inline citation, but which doesn't (Use your common sense: the statement "The sky is blue" probably does not need a citation).
Template:Infobox person - A generic infobox for people (Like the one you see on the right)! :-)
Template:Refimprove - Use when an article contains at least one reference, but they're not enough!
Template:Technical - Add to talk pages of articles that you feel are too technical for a broad audience to be able to understand
Template:Underconstruction - Put this template on pages you are currently busy editing, if you intend to take a really long time editing