If you know of something that should be listed here, please tell it the kde-i18n-doc mailing list,
kdedict by Matthias Kiefer: cgi script to store standard translations in a database and make it available via the web (there's also a demo available).
kbabeldict by Andreas Rizzi which is part of the KBabel package: creates compendium files and provides a convenient interface for searching them
Flexicon by Logi Ragnarsson: a database of words, each with an assigned language and description.
An index of on-line dictionaries and a lot of other resources can be found at www.yourdictionary.com.
Finally a tip: in the German team they are not using their mailing list but a web forum for discussing difficult translations (one term per thread) which has the advantages: (1) that everybody can jump in without the need to subscribe to a mailing list (2) and everything is pretty well documented and easy to find for translators to come.