Tutorial Display Last Tweet
The code below no longer works as-is with the Twitter API update to 1.1 as of 2013-06-11. The 1.1 API requires oAuth which requires a server side component. Here's a PHP way to interact with the new API. Googling around will find you many other libraries and such. There is an all-JavaScript method for getting tweets, but it may be a bit of a loophole that you may not want to count on.
$.getJSON("https://api.twitter.com/1/statuses/user_timeline/chriscoyier.json?count=1&include_rts=1&callback=?", function(data) {
$("#twitter").html(data[0].text);
});
Make sure you change username.json to your actual username and #twitter to the actual selector you wish to replace the contents of.
You can play around with this on this CodePen (deprecated).