I found this online grammar correction plugin by chance, as Opera's built in spell checker is pretty useless and I had to keep changing to Chrome when writing articles for DCT.
It's a free plugin, although it does offer a paid premium upgrade and I now use it across most websites. It works in most forums and email clients, but unfortunately not in WoltLab software, which is what this forum uses.
I've raised the issue with Grammarly support and they're looking into the matter, so hopefully we'll be able to use it at some stage in the near future.
It's very useful for both spelling and grammar and can be set for British, Australian, Canadian and American English, so it cover both aspects of our writing - spelling and grammar.
I'll keep this post updated if I hear of an update so that it works on this forum.