Watered down beer will add nutrients and nitrogen to the soil to give your plants a boost.
Give your compost a jump start by dumping beer on it. The yeast and sugar in the beer feed the beneficial bacteria in compost to really get it cooking.
Add some stale beer to your wasp trap to keep these stinging brats away from you while gardening. Here are directions from Instructables to make your own wasp trap from an empty soda bottle.
Dig a shallow hole in the dirt and set a pie tin or similar shaped plate in the ground. Fill with beer. The fermenting yeast smell will attract the slugs which will crawl in and drown. You can use fresh beer for this one if you happen to have some on hand that you don't like.
Clean your tools:
Carbonation in beer can help remove rust from your garden tools. You'll need a fresh beer for this one too, but simply soak them for about 10 minutes then buff with a cloth.
Slice an overly ripe banana into a shallow dish. Drizzle with blackstrap molasses and add beer almost to the top of the banana slices. Hang from a tree.
Add a few tablespoons of stale beer to a quart of water once a month. Water houseplants as usual.
Bonus in house use....
Fruit fly trap:
Place a cup of beer on the kitchen counter. Cover with plastic wrap and secure with a rubber band. Make holes in the top with a toothpick. Fruit flies will crawl through the holes to get to the beer but get stuck inside and drown.
If you happen to have any more stale beer left, you can water those brown patches in your lawn with it. The acids will help kill off the fungi that causes some brown spots, plus the fermented sugars will feed the new grass. You might want to make sure the neighbors aren't out first. You'll look mighty funny trying to get your lawn drunk!