Nothing new here: just a few links to various online word cloud generators.
Wordle is the best-known and most widely used application.
Tagul Clouds allows you to put your words into a custom shape, to use a variety of fonts, and to orientate the words in a variety of ways. Here is an example drawn from my recent article about the Times Festival of Education:
Tagxedo is a similar package, but has a different variety of shapes on offer: it would lend itself well to Biology and the Humanities as it includes the outlines of countries, plants and animals.
VocabGrabber takes a all this a step further by analysing the text you want to feature in your word cloud.