I'm yet to come across a spellchecker that works exactly how I'd like. This is mainly because of far too many missing entries. A (seemingly) easy solution would be to use the entire contents of Wikipedia with each word/term/phrase sorted A-Z (and all duplicates deleted). That would instantly make a spellchecker FAR better than any existing one.
And when it comes to spellchecking letters written in a word processor, then pretty much every line of every address is queried by spellcheckers, including postcodes/ZIP codes. I live in the UK and years ago you could buy a DVD from the Royal Mail of all the addresses in the UK. I don't know if that's still the case, but if all the addresses in a particular country (or countries plural) could be imported into a spellchecker that would eliminate this annoying spellcheck problem.
