If possible one that supports at least spell checking:
The plugin from Microsoft's Mikhail Arkhipov does HTML and Comments, I don't believe it does C# strings, though. I use the Agent Smith plugin for ReSharper for that.
If you're using ReSharper, there's a free addon for it called Agent Smith Plugin. One of its many features is a built in spell checker, that allows fixing the spelling mistakes using the ReSharper shortcut key, Alt-Enter.
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