In comparing wordle to wordsift, I found I liked wordle better. Students can keep a running list of related words for a topic. They can then enter all their words, save as a jpg. then use Adobe Acrobat to embed a link on each word to show its definition.
Another feature I liked was the way key words are emphasized, repeated words allow for a visual connection and the site allows to randomly sort and resort words for a variety of finished products.