In addition to the traditional tag cloud availabe in Koha, there is a way to generate clouds for popular subjects within Koha.
The Author/Subject Cloud cron job is used to help with this process. This cron job sends its output to files.
This means that you can produce clouds for authors, collective author, all kind of subjects, classifications, etc. And since it works on zebra indexes, it is quick, even on large DBs. Tags clouds are sent to files. It's up to library webmaster to deal with those files in order to include them in OPACMainUserBlock, or include them into their library CMS.
Some libraries even send the file into a Samba shared folder where webmaster take them, eventually clean them a little bit before integrating them into navigation widgets or pages.