SCOWL (Spell Checker Oriented Word Lists) and Friends is a database of information on English words useful for creating high-quality word lists suitable for use in spell checkers of most dialects of English. The database primary contains information on how common a word is, differences in spelling between the dialects if English, spelling variant information, and (basic) part-of-speech and inflection information.
SCOWL itself is a compilation of the information in the database into a set of simple word lists that can be combined to create speller dictionaries of various sizes and dialects (American, British (both -ise and -ize), Canadian and Australian).
Premade dictionaries are available for Hunspell, Aspell, and as plain wordlists. If none of those dictionaries are suitable for your needs a simple web app is available to create a customized wordlist.
A simple web app is also available to check if a word is in SCOWL. This app also assigns a score that indicates if a word should or should not be considered for inclusion based on its frequency in Google Book's corpus (1980-2008). In addition, a report sorted by frequency is available that also looks for similar more common words to help determine if adding the less common word might cause a problem.
VarCon (Variant Conversion) is the primary source of spelling differences and variant information. It can also be used to convert between American, British, Canadian and Australian spellings.
Alan Beale 12Dicts is another main source of information in SCOWL. It contains a variety of lists, of different sizes and characteristics that are used by SCOWL.
The 2of12id.txt file, in the alternative version of 12Dicts, is the primary source of part-of-speech and inflection information, however it is limited to common words. AGID contains more words but also likely to contain more errors.
The SCOWL collection contains many others pieces of information. Most of these are in the forms of other word lists.