Babiloo is free and open source. It supports formats of other dictionary applications as well so you can import a myriad of dictionaries.
- Runs on most of the platforms, Windows, Linux, MacOS, Nokia S60
(>1.9).
- Supports dictionaries in SDictionary, and StarDict formats.
- HTML displaying for the supported dictionaries.
- Able to download more dictionaries within the application.
- Translated into more than 30 languages

StarDict is another popular one:
Note: In 2016, this message appears on the old StarDict site:
The original StarDict project has recently been removed from
SourceForge due to copyright infringement reports. Most of the files
were lost with the demise of the project. The project has re-emerged
here since then, though the legal issues may never have been resolved.
StarDict hasn't seen any active development for many years, while the
world around hasn't stood still. The following is the list of
alternatives appeared during that time. You are advised to take a look
and try them:
- GoldenDict - designed as a StarDict successor, it supports its format, lots of other formats (Lingvo, Babylon etc), features tabbed
browsing, morphology, live pronunciations, Wikipedia and so on. The
program is constantly being improved and worked on.
- Babiloo - a free open source software developed to read offline dictionaries. Runs on most platforms. Supports StarDict and
SDictionary formats.
- LightLang - another system of electronic dictionaries (Linux only).

JaLingo is another one. It's written in Java making it highly portable:
