It comes with an American-English dictionary containing more than 110,000 words.
Microspell also seems to be US English with an option to import Ispell word lists.
"Includes comprehensive American English dictionary. MicroSpell includes a thoroughly checked American English "main" dictionary and several sample user dictionaries, including one with many computer-related terms. Other dictionaries based on public domain lists are also available. MicroSpell can also import Ispell word lists which cover British English, French, German, Spanish, and other languages. (We are seeking other non-English word lists; if you know of any good ones, please let us know.)"
Jarte's spell check has an option for US or British English. Both are included in the download. You can add your own words too, of course.