Semagic has been without a decent spellchecker for a while, and lots of clients don't have spellcheckers at all. I think it'd be nice if there was an interface available by HTTP to the spellchecker configured on a LiveJournal server, since that would unite all spellchecking in one place, and remove all of the yucky licencing issues with that windows spellchecker library
Issues to consider are how it should handle multiple languages (which requires that the server support multiple languages for spellchecking, but this is desirable anyway) and how best to return the results in a format which the client will be able to use to locate and flag the errors in an intuitive way.
This needs to be restricted to require the details for a valid LJ account, so that other software does not use LiveJournal.com as a public spelling service, but this doesn't cause a problem since clients already process and send these details.
This would be pretty easy to do with the XML-RPC interface, but not so with the flat interface which would require a mess of junk to be sent to the client. I'd like thoughts on a more flat interface for the spelling checker so that clients do not have to depend on an XML library.