I frequently find myself writing blocks of end-user facing text that are inserted into various Workflow UI components. There's no built-in spellcheck capability, so my mediocre typing accuracy has a bad habit of showing up far too often.
I'm sure there are ton of these sites, but this is the one I've been using for a few weeks now. It's a painless experience to paste in the text, get corrections and copy the results back out. I've only been using the English dictionary, but supposedly it has support for 27 languages.
A practical example: