Translate.Net is free and open source client for many language services available online (translators, monolingual and bilingual dictionaries). 45 sites, 57 services, 70 languages, more of 4000 translations directions.

Services:

• Google dictionary and Google translator
• 9 DICT servers with 94 dictionaries
• Wikipedia and wiktionary
• Yahoo Babel Fish translator
• SYSTRAN translator (used by Altavista Babel Fish)
• Merriam-Webster dictionary and thesaurus

Full list of languages\services available here

Features:

• intelligent switching of translation directions based on automatic detection of input language and keyboard input language (keyboard layout)
• system hotkeys by keyboard(Ctrl+C+C and\or Ctrl+Ins+Ins) and mouse (Alt+Right mouse button) or custom mouse\keyboard combination allow to activate Translate.Net and perform translation from any program easy
• available portable version
• ability to create user profiles - custom set of services
• history of used language pairs which sorted by last use time