The European interinstitutional style guide:
Mandatory reading and currently in the process of being expanded to cover 23 languages, this database is intended to assist anyone involved in writing documents (hardcopy or electronic) working in any of the European institutions.
Microsoft's very own language portal:
The perfect tool if anyone who needs to conduct an online search in the terminology database and localisation glossaries compiled by Microsoft. Also available (for download): Microsoft's style guides.