I think that the best open source off-line dictionary is Stardict. It’s absolutely free, it’s very user friendly and, of course, it supports a huge amount of dictionaries which allow to translate words and phrases into many directions.
Few will deny that there is no better Russian-English and English-Russian dictionary than Lingvo, but shame on it’s developers — it’s for Window$ only. This problem can be solved rather simply by using Lingvo dictionaries with Stardict… Here are links for EN-RU and RU-EN dictionaries:
I use them by myself at my Ubuntu laptop and they are very easy to install. Just install startdict by command
sudo apt-get install stardict (for Ubuntu and Debian) or
yum install stardict (Fedora, CentOS, RedHat etc.). The unpack them into
/usr/share/stardict/dic and restart Stardict. Here are results: