I love this post from wikiHow on how to make fake blood for Halloween. This is the best mix of recipes I've seen so far because it shows you a non-toxic version, a toxic (more authentic) Hollywood movie set version, an edible version for cakes, splatter blood (ha!) and even blood for art.
Here's my friend Susan's version of non-toxic blood that I used several years in a row for my zombie prom Halloween costume:
- Arrowroot powder (or corn starch, but Arrowroot makes for a less clumpy mixture)
- white corn syrup
- water
- basic red and blue food coloring (green works too if you don't have blue)
Start with a cup of corn syrup in a bowl. Stir in a tablespoon of Arrowroot powder or cornstarch. Slowly add in water (up to 1/3 cup) until you get the perfect consistency of blood. This is your base.
To add the dye, add a few drops of red food coloring, stirring it in until you have a bright red color. Add a couple drops of blue or green food color to make the "blood" look more authentic. Set aside to thicken, then apply to face, clothes or body. Warning: This fake blood will stain clothing!
Best Halloween hair and makeup
Image: Photodisc for Getty Images

Comments
Update: I actually made this homemade fake blood myself for Halloween this year and it was a HUGE hit at the parties I attended. Many people said it seemed more authentic than the fake blood you can buy in stores. If only they made brown food coloring because blood dries brown-ish and I should have toned down the bright red a bit more with the blue.
Not to mention, this is EXTREMELY sticky!!!
If you mix black and yellow to the red you will get that brown effect ; )
if you add a few drops of yellow to a small amount of the blood and mix it then mix it in with the blood after you put it on your sefl it will give off the brownish look (be sure to use more blue when you mix the small amount of fake blood with the yellow) (( PURPLE + YELLOW = BROWN))