It is a free standard. There are free servers and clients.
The project has amassed an impressive list of dictionaries that appear to be available to whoever wants to use them (some sort of GNU license on the dictionary databases).
You can query dict.org and indeed, it will look up all sorts of words.
This firefox addon client is pretty effortless to install…
But you’ll need to edit firefox’s config to point it at a custom dictionary server.
The raw format is XML. Here is an example (scroll down). So you’ll need an specialized editor to get your dictionary into a suitable format (don’t tell me I can edit xml in notepad, to that I say, you can write youtube videos in a binary editor!).
It is a bit tricky to grok, it’s a file format, an client API, a protocol over TCP-IP, it’s a server app. This isn’t yet something that all mere mortals can cope with.
To host a server, you need to give up an port on your domain. WTF? My host–and I have a virtual machine– has only so many ports open, and most of them I have to leave as is.
The servers seem to be better supported on Linux although there are ports.
Ordinary HTTP seems to have won the protocol wars for all media, including dictionary lookups. Look at this firefox plug in.
That plug in is willing to talk to any website that takes lookups in the form of a URL, where as a DICT client can only talk to DICT servers, and there are fewer of those than just ordinary websites with a “search dictionary” form.