You have a lovely pair of avocados on the counter, but they're not quite ripe yet. If you want to make them (or most fruit) ripen faster, put them in a loosely sealed paper bag. As explained by food site Chowhound, most fruits release small amounts of ethylene gas. Reabsorbing this gas leads to the physiological changes that we know as ripening. By placing your fruit in a paper bag, you trap more of the ethylene gas in place, making the ripening process happen faster:
Putting the fruit in a loosely closed paper bag traps the ethylene gas inside and accelerates ripening. Don't seal the bag too tightly or use a plastic bag, however -- if you trap too much moisture, mould may grow. If you really want to ripen things in a hurry, put an apple in the bag with your other fruit -- they produce a lot of ethylene gas.
If you don't have a paper bag handy, you can use nearly any container that reduces the amount of air around a fruit without sealing it entirely -- one Reddit user put an avocado in a stubby holder, for example. However, you don't want to seal the fruit in something airtight like Tupperware. This can trap moisture and cause mould to grow.
Why Does Fruit Ripen Faster in a Paper Bag? [Chow Hound]