I used this idea for my wedding invitations.
Here are the files for my invites with a quick tutorial here.
I would love to receive one of these invites from The Ghostess' annual Halloween Party at Dead End.
Some other ideas (click on image for source link).
Return Address Goodies!
These could be complimented with a great Halloween postage stamp.
"Boo!" stamp by Noteworthy Collection
Haunted House stamp by PSA Essentials
Jack-O-Lantern stamp by Noteworthy Collection
Check out this post from Brooklyn Limestone blog for some other great ideas.