I think the most useful Java aplication for your mobile is maybe ... dictionary.
Of the best are:
LIVE Dictionary No1
Translation Java dictionary for your mobile phone
MSDict Concise Oxford English Dictionary
The Most Popular Dictionary Worldwide
An OXFORD English dictionary and thesaurus with over 240, 000 words and definitions, pronunciation and grammar.
MSDict Pocket Oxford Duden German Dictionary ENGLISH-GERMAN
Recommended by Teachers and Academics around the World
MSDict Concise Oxford Thesaurus
An OXFORD English Thesaurus with over 365,000 alternative and opposite words to make your writing or speach more interesting.
Viren`s Dictionary of the English language has over 143000 entries.
Free 19 languages (English too) mobile eBook - Go Bible
Go Bible is a Free Bible viewer application for Java mobile phones (J2ME MIDP 1.0 and MIDP 2.0)
Skype for [WiFi] Mobile Phone and Pocket PC
Google Mobile — Mobile Web Search
Java™ Platform - Java Micro Edition - Java ME