Free Halloween Sounds (free) is just plain fun, what it is is fifteen free spooky sound effects (bats, ghosts, chainsaws, creaky floors, etc) and a spooky soundtrack you can play in the background. To kick it up a notch just get a mini jack to 1/4" stereo jack and plug it into your sound system for Halloween and really give the kids a fright! I've been playing with this thing so much even my son is getting tired of it, and he's 7!
The next app is Halloween Countdown (free), complete with skeletons and a hauntingly good background theme, and of course the countdown. Now there's not much to this app, it's just a countdown timer, but so what, when you're out trick or treating with your kids, or if your a kid, this is just one of those mindless but fun apps to get you in the mood.
Finally I have to recommend you try Halloween PhotoCraft (free), which basically allows you to overlay some stock images of fangs, blood, spiders, bolts for your neck, and some scars onto anyones photo. I've been playing with this one non-stop too and it's loads of fun because you can not only save your creations but e-mail them, send them direct to Twitter or Facebook!
For a really good fright check out my new Halloween video. It's not as much a review as an overview and "Dexter" theme.