Google Translate Desktop is a free tool that makes it easy to translate blocks of text from one language to another.
Simply copy your source text to the clipboard, launch the program, and it will automatically import your selected words. Then choose the destination language, click Translate, and the translated version appears almost immediately.
You have more text? Not a problem. Just it to the clipboard, and Google Translate Desktop will detect this, automatically translating it to the language you selected last time (although you can manually choose another, if you prefer). Click the Copy button and the translation will be placed in the clipboard, ready for you to use in a document, an email, on the web, wherever you like.
Please note, despite the program name, this isn't a Google product: it just uses the Google Translate service to carry out the translations.