As the title says, I think having a widget for your email inbox (at least unread emails, but this could be a setting) in a Blackberry-like fashion would be very helpful.

It could be a widget in the form of the calendar widget, maybe each item being clickable and opening the corresponding email inbox or maybe the email item itself.

Features I would like to see:

1. It'd probably be useful to have some sort of sorting option, allowing you to sort the emails in the widget by either date or unread state (or whatever you fancy).
2. A resize option would probably be desirable, I for one would dedicate an entire desktop to this but I'm sure not everyone would.
3. Setting deciding how many emails to show would probably be nice.
4. Maybe a setting to show the first line of the email like the actual client, though this isn't top priority.
5. Some indicator to show what email account it belongs to, but again, not extremely high priority (you'd see this when you'd open the mail anyway).

Brainstorm link.