Replying with Attachment...or Not.
How many times have you had to send that second, embarassed email, saying, OK, this one REALLY has the attachment I was talking about in that first email.
Wouldn't it be nice if your email program asked you about sending an attachment, or had a Reply with Attachment that made you pick the attachment first before you wrote your email? It turns out someone at the Why Not? site has thought of this already.
In fact, the Why Not? Site has this and other interesting ideas, many in the spirit of usability.