Is there a Mail plugin to let me view unread emails first?

Solution 1:

In Mac's mail app you have got the option to create a 'smart mailbox'. This allows you for example to only view unread emails. It can even do that over multiple mailboxes.

Sample rule for unread messages of today (taken from makeusof.com):

If you just want to have the unread messages always on top, there's a sort option for that in the menu between Edit and Mailbox (probably called View).

Solution 2:

In addition to Vincent's answer, you can be more creative with your Smart Mailboxes, but since you're in Mountain Lion, you can sort any Smart Mailbox by Unread/Read status.