Sign up as a test candidate for the Aperture Science Computer-Aided Enrichment Center by purchasing this fairly cool Portal hoodie from ThinkGeek. It's 100% cotton, with two front pockets, ribbed cuffs and bottom, and the zipper pull is a silver-colored Companion Cube! It comes in sizes S through XXXL for $59.99, and can be ordered directly from their website.
Now if only it actually came with an Aperture Science Handheld Portal Device... But I guess that would jack the price up significantly. Oh well!