Some form of spell-checking would be really useful. Perhaps not an automatic one, but at least one that works on selection of a block of text.
Michael Coyne commented
Be great if this could be taken one step further and have "code aware" spell check. This way it'll only spell check things like variable and function names etc etc.
aspell would be the solution here. I know it has an interface for other apps to get at it's spelling goodness, just not quite sure how (and if there's a ruby layer already existing for it)