/usr/share/dict/words but in the browser, it isn't viable to use local files.
I uploaded a copy of my local file as a Gist so it can be easily retrieved with a standard AJAX call. Since the file of the format in Unix simply separates the words by a line break, I also transformed the data into a JSON array, so it could be easily parsed. It is a large file (over 16000 lines) and it takes a while to transfer, so I'd only recommend getting the whole list if your script really needs it.
Here's the script I used to convert the file to JSON, for the curious:
with open('/usr/share/dict/words') as infile:
words = infile.read().splitlines()
with open('words.json', 'w') as outfile: