I just whipped up this tampermonkey script up after getting mildly annoyed with having to navigate to a user's profile page to view his or her karma.
This allows you to hover over a username to make an XHR request to the profile page. The karma will be parsed out and inserted as a new <span> element directly to the right of the username.
On Chrome, just install the tampermonkey extension and then this script. I'm sure it would work on FF with Greasemonkey too with little or no modification.