12dicts is a collection of English word lists by Alan Beale. It differs in several important ways from most of the other free word lists you can download.
The 12dicts lists are oriented towards common words. If you're looking for myriads of archaic, scientific or computer jargon words, you should look elsewhere.
The 12dicts lists have been rigorously checked for errors. (This is not to say that they are error-free, merely that enough care has been taken that errors are rather infrequent.)
12dicts contains a variety of lists, of different sizes and characteristics. One size does not fit all. Because each list has different characteristics, I do not recommend combining them, except as noted below.
Originally, 12dicts was composed of lists derived from a specific set of 12 source dictionaries. In addition to these "classic" lists, Version 5 of 12dicts now includes lists derived from other sources. It would perhaps be appropriate to rename 12dicts to something more generic, such as BAWL (Beale's Assorted Word Lists), but this has not been done so in order to preserve continuity.
The Unofficial Alternate 12 Dicts Package is the one used by SCOWL. It is based on version 6.0.2 of the official 12Dicts package. It contains almost all of the information originally found in Version 4 of the 12Dicts package but in a different format as well as a good deal of additional information. However it is not meant as a replacement for the official 12Dicts package. It simply offers the information in a different way.