Although this spider & web vinyl decal ($22) from Vinyl On The Go would be PERFECT for Halloween, it's so cool, I'd be tempted to leave up all year.
If arachnids aren't your thing, how about this bird in cage ($15)? Very Raven-y, no?
Or how about this moody tree ($22)?
You can find these decals and many others at VinylOnTheGo's Etsy shop. Great stuff!