QTranslate is a free translator software for Windows.
Main features:
Translate text in any application that supports text selection (Google Chrome, Microsoft Word, Microsoft Outlook, Acrobat Reader, Skype, IE and etc.);
Back translation;
Replace the selected text with its translation;
Instant translation;
Spell checking;
Text to speech synthesis;
Word suggestion/autocomplete (Ctrl+Space);
History of translations (Ctrl+H);
Virtual keyboard;
Portable version.