Tag clouds are best when they are relevant to the user's particular interests. For example, a tag cloud that shows popular tags of the last few days is likely to be more interesting (to the breathing) than a tag cloud that shows popular tags "of all time." Also, if you filter for recent activity, the content of your tag clouds will change every day, rather than remaining static.
Tag clouds can also be used to accompany and annotate search results. When a user searches for a particular tag, you can display a cloud showing related tags. This result will be much more interesting to the user than a tag cloud showing only the most popular tags on the server.