The Freelang dictionary is one free program for Windows. It is very easy to install and use: after installation select one language from the list. Then you can download it and add it to the program. 
After installing the Freelang main application, you can download and install as many languages as you like. The program lets you browse lists, search for a word, add or modify an entry, and learn new words from a personal learning list.
All keyword lists are bilingual (to and from English). The application also has other combinations (such as from and to French or from and to Spanish). The program runs on all versions of Windows. It's free, but the lists are protected by Copyright for their editors.
If you can't download the dictionary or want to rate it first, you can use it online version (works for all platforms: Mac, Linux ...).
Currently, 270 dictionaries are available. These include Greek.
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- Greek-English dictionary at the online bab.la language portal
- G. Babiniotis - Small Dictionary of Modern Greek Language for iPhone
