Weighted Companion Cube casemod. Here's a detailed thread on how it was made.
Which makes this a good time to post the two recent and very funny Portal 2 ads:
(Did you know that bloggers were paid to post those videos? I'm not sure why the company went to the trouble because the ads are plenty good enough that bloggers should have wanted to post them for free.)
Finally, Think Geek has a variety of Portal goods, including this Portal 2 poster: