Interesting tag cloud implementations
Form over Function disease!
The tag cloud concept is to show tags and their frequency, typically with proportional font size
The Cumulus 3D and similars are just special effects and do not show frequency very well, or at all!
Stick to the basics and your users will love you
Colour coding tags into groups is the simplest improvement without distorting the nominal function. Show the colour legend if possible
I personally also like colour intensity to indicate the recent usage of tags. A popular tag that has no recent posts will be in a large font size, but in a low colour intensity so that it fades away and looks less important. e.g.
The most interesting tag clouds are probably those by Jeff Clark.
Shaped word clouds
(source: neoformix.com)
Word Portraits
(source: neoformix.com)
Clustered word clouds
(source: neoformix.com)