It's too late to safely mail your ballot in, but you can drop it off at any library, at the Multnomah County Elections office (SE 10th and Morrison), or at unofficial spots like either Obama headquarters (NE 15th and Killingsworth or SE 30th and Division), among other places.
And if you're still one of those "undecideds" when it comes to ballot measures, check out this guide from the kids at the Bus Project. It lines up the endorsements from several sources, including the Mercury, to help you make up your mind.
(Click on that to download the PDF!)
P.S. - You should totally go trick-or-voting with the Bus Project tomorrow. It's the biggest get out the vote event in town!