Similar to Gmail priority inbox, is there a Mail 6.3 (OSX Mountain Lion) plugin that lets me view unread messages first or sort them so they're at top of inbox?
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).
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.