Invest your money – Grow your Cash! – Acorns
Invest your spare change!
This app will help you earn money like a traditional retirement account. Deposit the minimum (I believe it is now $100), set if you want it conservative, moderately conservative or aggressive, and watch it grow! You then can link your bank/credit card and have it round up to $1.
Once it does that you will start adding to your account. You can withdraw at any time. There is a small fee attached but it takes it right out of your earnings. On average, I deposit $22 a month without doing anything and have had gains of about 20% since I started.