Maple syrup - Replace the vanilla with an equal amount of maple syrup. Any changes in flavor will be subtle.
Almond extract - Swap out the vanilla with half as much almond extract.
Vanilla milk - Have vanilla-flavored almond or soy milk in the fridge? Use it to replace the vanilla measure for measure.
Rum or brandy - If you aren't trying to avoid the alcohol, try an equal amount of rum or brandy.
Vanilla Bean - Scrape out the seeds from half of a vanilla bean, and use them as a stand in for one Tablespoon of vanilla.
Vanilla Paste - Replace the vanilla extract with an equal amount of vanilla paste.
If you have more than one of these substitutes on hand, choose the one that seems to be the best flavor match for the recipe that you're making. If you don't have any of these things on hand, simply leave the vanilla out, and continue on with your recipe.
Want to learn how to make your own vanilla extract? Here's the recipe: