You may have noticed a Dictionary app called Colordict in the Market.
It seems to have only six available dictionaries.
Well, in fact this app reads Stardict Dictionaries, Encyclopaedias, and many other reference books on Starldict format, and there are hundreds of them available for free.

Check and see for yourself.

Just need to know how to make it work in a Droid device, and that's quite easy. Follow these steps:

1. Choose your Dictionary and download the TAR compressed file to your PC.
2. Extract the contents (I use IZARC).
3. Rename the to XXX.dict.gz and decompress it (must use WinGZ),
resulting in a decompressed XXX.dic (you don't need the gz file anymore).
4. Copy all the files extracted in step 2 plus the one replaced in step 3 to
\\sdcard\dictdata\ on your device.
5. Open Colordict and let the app index the new dictionaries. Done.

If you prefer the interface, Wordmate (also available at the Market) does the same thing.


Some links: 
Bidirectional english:

Other english dictionaries & encyclopaedias: