I think it's a bit language-specific. For example, many websites that offer Japanese <-> English lookup use the free EDICT: http://www.csse.monash.edu.au/~jwb/j_edict.html

You could also check out the list of free dictionaries included with a translation app like Lingoes and see if the source data files are available: http://www.lingoes.net/en/dictionary/index.html

The Vicon series of dictionaries covers a lot of languages, but I have no idea where they come from. You may also need some logic on a per-language basis if you want to consider conjugation, plurals, etc., since these dictionary files don't list all alternative forms.

I find that I quite often simply look up a word on answers.com when I need a bi-lingual translation. They have a rather long list of dictionaries incorporated into their dataset. I did a quick digging around and found that they apparently got them from Wizcom Tech [http://www.wizcomtech.com/] See: http://www.answers.com/library/Translations

I'm actually also building a language webapp atm, but I'm lucky enough to have the excellent EDICT dataset from Jim Breen, which has already been linked here. If I were expand beyond Japanese, I'd definitely start out looking for other such datasets.

Good luck

*nix systems have a dictionary daemon dictd that can talk to many dictionaries. I remember running a Latin-English/English-Latin dictionary on my Debian system years ago.

You could try googling for that and the FreeDict dictionary databases.

Are you looking for mono-lingual or bi-lingual dictionaries ?

I'm pretty sure there are freely available mono-lingual dictionaries available for many languages but I don't know about bi/multi-lingual.

Thanks, I should have specified - I'm looking for bilingual dictionaries. They don't have to be free, as long as the price isn't exorbitant - I should be able to charge for the webapp (or at least a freemium model), so I don't mind a modest investment.

Take a look at Wordnik. It has a REST API for dictionary lookups and more.

http://docs.wordnik.com/api

Would WordNet or Wiktionary work for you?

They might, thanks! I'm mainly looking for bilingual dictionaries, but something like Wiktionary may not hurt as a bonus feature - I hadn't really thought about it.

Wiktionary does provide equivalent terms in other languages. For example: http://en.wiktionary.org/wiki/trawl . The problem is that this feature is no where near as ubiquitous as it needs to be for it large-scale use.