This past Halloween, we talked about a few projects (like this one and this one) that we thought were cool. Of course, if you are anything like us (and I'm guessing you are), building a cool Halloween costume can be a year-long endeavor. So here's something you might want to get started on - Arduino Animatronics.
Ever dream of having a Predator cannon as part of your costume? Check that out! This tutorial from Instructables shows you how to implement a bit of awesome animatronics into your costume.
They also have an awesome video of the Iron Man repulsor (I admit, I'm a big Iron Man fan). If you haven't already poked around the Instructables website, you should definitely do so - they have a million tutorials about pretty much everything you might want to build. Try a search for "Arduino" and get at it!