GoldenDict

There are a lot of third-party dictionaries the program can work with. Here we list some common sources for them.

A due disclaimer: no dictionary files are hosted here on this site, except for a couple ones which are well known and which probably don't actively infringe on any copyrights. If you download any dictionary content referred from this page or from any other page on this site, it's solely you who are liable for any possible arising copyright infringements.

Unfortunately, much of the dictionary content is non-free, and this area is generally known to be very murky. Therefore:

YOU USE ANY LINKS POSTED HERE SOLELY ON YOUR OWN RISK

We recommend to encourage dictionary content creators by buying their dictionary content. Buy it, then throw away their junk program and use your legally bought content with GoldenDict instead.

Now for the links.

• Babylon (.bgl) dictionaries

  Those can be downloaded here, from the official Babylon site. If instead of .bgl files you are getting .exe installers, use 7Zip to extract them (use full version of 7Zip, the one which supports CAB files). The .bgl files would be inside.
  Some dictionaries are free, some are provided on a try-and-buy basis. We encourage you to buy them after you've tried them if you've decided you're going to keep using them.

• ABBYY Lingvo .dsl dictionary files

  Since Lingvo is a product of a Russian company, most sites here are in Russian. Nevertheless, many of the available dictionaries have nothing to with the Russian language themselves, so you can try finding what you want even if you don't read Russian.

  As a starting point, there's a large catalogue here. It lists virtually all the dictionaries ever made for Lingvo, but no files are hosted there -- only links at best. Use it to find whether or not the dictionary you are searching for indeed exists -- and sometimes the download link is present, as well. Also notice that this site has a list of other sites with .dsl dictionaries, here.

  Quite a lot of nice dictionaries are hosted here, all are readily downloadable.

  Here's another site devoted to Lingvo dictionaries' creation.

  And this forum thread is all about creating user dictionaries in .dsl format. It has a lot of content created already, and the members are very active. You will need registration in order to read it, though.

  Some Lingvo dictionaries are only available in a binary .lsd format. Those are .dsl in compiled form. There's no official way to unpack them back to .dsl, though some unofficial ways do exist. If you absolutely need to convert some .lsd back to .dsl, you can try the forum thread mentioned, or just try googling.

  Our official position on the decompilation of the Lingvo's official dictionaries is as follows: We believe that if you legally own a copy of the Lingvo program, you are eligible to be able to use your dictionaries under any other program of your choice, and therefore you should be allowed to decompile those dictionaries in order to get them into an interoperable form. However, this act may be expressely forbidden by the Lingvo EULA. Whether this is legal or not is questionable. Consult your lawyer if you have doubts.

Instead of fighting with alternative programs which allow using your dictionary content interoperatively, we urge you to make an infrastructure to sell it for use with the program instead. We believe in a possibility of a market for the dictionary content, as opposed to the market for dictionary lookup programs. If you would want to sell your content for use with GoldenDict, we could help you integrate your shopping place into the program. You may also choose to use DRM protection schemes. However, all the code implementing them would still be available under the GPL. We believe that this would still be a sufficient protection, since 1) while it would be possible to circumvent the protection by modifying the source code, the majority of users won't be thinking about that. If one can't easily copy another person's dictionary, it is just easier to buy it than to tinker with some cryptic source codes. Also, 2) there are laws preventing to break copyright protection schemes, and 3) there's a general copyright law applying to dictionary content.
The licensing should be fair, and non-discriminating to other possible sellers, or to any other third-party content. Think about it.