What I often miss when using spell checking are the names of cities, names of people or special terms. I know there are custom wordbooks and sometimes wordbooks for special terms. But the efforts for this are kind of "to big". Therefor I want to suggest to create additional wordbooks containing these words which have been automatically parsed from wikipedia and similar open content institutions. Wikipedia has the advantage that things are categorised very much. So it should be possible to extract names and cities. I would also assume that names of people and cities too are at least if they appear in a article name are spelled correctly (to be sure one could (if possible) only include articles in this parsing which are older then a month or so).
I'm working on a project similar to what you describe. It extracts words from the Icelandic Wiktionary project and merges with another wordlist. Other languages and sources, e.g. wikipedia can be added with ease. Here is a link to the project: http://launchpad.net/hunspell-is .