I've rounded up five of my favorites--some for kids, some for grown-ups. Take a look:
123 Glow HD Coloring Book, Halloween Edition A killer coloring book with 14 Halloween-themed pages (out of 48 total--others include autumn and dinosaurs). It's educational, too, teaching things like numbers, letters, and colors in your choice of four languages. It's best on iPad, but also works on iPhones. Price: $1.99.
All-In-1 Zombiebox "Tis the season for zombie-whompin!" This 99-cent app serves up 10 zombie-themed games, including Zombie Pizza, Zombie Karts, and the impossible-to-resist DefCon Z.
Carve It! Pumpkin Carving Gutting a real pumpkin is so messy. This app lets little kids "carve" one of three different-size pumpkins, light the pumpkin when it's done, save and/or share the results, and even play a game of Pumpkin Bungee. Price: 99 cents.
The Legend of Spookley the Square Pumpkin The popular children's story comes to life in this interactive e-book, which offers both autoplay and read-to-me/read-it-myself modes. The $2.99 app comes from Oceanhouse Media, makers of the popular Berenstain Bears and Dr. Seuss e-books.
SpookyTones Want to hear a scream when you get e-mail from the boss? How about the sound of bats for mail from your crazy uncle? SpookyTones (99 cents) lets you assign custom Halloween-themed "ringtones" to specific senders. If that sounds familiar, you might be thinking of its more generic predecessor, MailTones. Either way, it's cute and clever fun--though you have to be willing (and able) to forward copies of your mail to the MailTones server.
Have you found any other must-have Halloween apps? If so, haunt the comments with your picks.
Late addition: Don't miss Angry Birds Halloween!