Wordle is a free word cloud generator. You can paste in some text or just add the url to any website or feed. From there you can specify the colours, fonts and layout. The clouds give greater prominence to words that appear more frequently in the source text, so they will be larger in the final artwork.
Unfortunately you can't specify the shape of the final cloud, but it's great for brainstorming, finding out about your blogging habits or just wasting a pleasant 20 minutes.