Here is one that I hadn't seen before, though it bears a date of December 16, 2015: by MyKey Technology, Inc. in support of neither party. This amicus brief (which appears to be filed in connection with Halo only, and not the companion case Stryker; not sure why) argues that the Supreme Court should read U.S. Patent Act section 284 as not requiring a showing of willfulness as a precondition to an award of enhanced damages; that such awards should be committed to the district court's discretion; and that they may serve both compensatory and punitive purposes.
For links to the other amicus briefs that I am aware of as having been filed so far, see here. Patently-O also published a post on the amicus briefs shortly before Christmas. I shall endeavor to keep readers abreast of the remaining briefs when they come in. According to Scotus Blog, oral argument in the two cases is set for February 23.