Many applications are let down by the quality of the 'writing' in their user interfaces: typically, poor spelling, grammar, inconsistent tone, and worse yet, "humour" are the usual offenders.

Are there good resources that can help developers to write UI messages that give a professional and positive impression to your customers, even when your code's going to hell in a handcart?

Thanks, all — Some great resources here, so I will CW this question. I'm accepting Adam Sill's answer because it's the one that (as a developer of desktop apps) I found most pertinent.