WeighWords is a Python library for creating word weights from text. It can be used to create word clouds.
WeighWords does not do visualization of word clouds. For that, you can paste its output into a tool like http://wordle.net or the IBM Word-Cloud Generator.
Rather than use simple word frequency, it weighs words by statistical models known as parsimonious language models. These models are good at picking up the words that distinguish a text document from other documents in a collection. The downside to this is that you can't use WeighWords to make a word cloud of a single document; you need a bunch of document to compare to.
D. Hiemstra, S. Robertson and H. Zaragoza (2004). Parsimonious Language Models for Information Retrieval. Proc. SIGIR'04.
R. Kaptein, D. Hiemstra and J. Kamps (2010). How different are Language Models and word clouds? Proc. ECIR.