All the “where to invest $100” articles telling you to buy CDs and open money market funds are dead wrong. Absolutely dead wrong. Where should you be investing $100? Look for promotional offers from online bank accounts (where you can generate some more income by referring friends and relatives), brokerage firms, and other financial institutions to get the most bang for your $100. Then, after you’ve earned enough through those offers you can start thinking about “conventional” investment vehicles.
Don’t have $100 you want to invest, consider signing up for a credit card that offers statement credits or gift cards  that you can convert to money to invest.
To learn more, plow onward!
1. Sharebuilder 
First step is to open up a Sharebuilder  account using a promotion code that will give you free money. Currently there’s a $50 bonus code 50GO28 if you deposit some funds and make a purchase. Stock purchases are a mere $4 but stock sales are a pricy $14.95. Let’s say you put in $50, make a purchase, get the $50 bonus money, and then sell your stock back at your purchase price. Net result is a gain of $30.05 ($50 less $4 purchase commission and $15.95 sale commission) which is a 60.1% gain on a $50 investment. Now you have $130.05 to invest. Some have reported being able to repeat this process over and over again on multiple promotion codes. I list all of the codes I know of on this Sharebuilder promotion code post .
2. Bank of America
BoA frequently has promotional offers for opening a checking account such as this $50 promotion (MYA) . The minimum you need to open most accounts is $100 and account fees are lower if you setup a direct deposit into the account. The “cheapest” account is the MyAccess Checking account which actually has no minimum and $5.95 monthly fee, waived if you have a direct deposit. Once you open an account, the $50 bonus will be deposited into your account within 50 days. Deposit $50, wait 50 days, get the $50 bonus. You can sign up for a BoA promotion every 60 days. According to their representative, the verification department runs a credit bureau report and a report from ChexSystems. After doing this once you’ll have $180.05 waiting to “invest.”
Now you’re on a roll… you can repeat these types of signup deals until you reach $250… (here’s a great list  of bank signup promotions)
3. ING Direct 
This promotion is for only $25 but it’s an easy $25. Just get someone who already has an ING Direct account to send you a referral (I have some available, hit up this self-serve $25 ING Direct promotion page  if you’d like one). All you have to do is deposit $250 and ING will send $25 your way, a quick painless $10% gain with no risk. In return, your friend will get $10. Now, you can give out twenty of your own referrals and start earning a little extra cash with no risk whatsoever. If you get all twenty, that’s $200 in your pocket. So, now you have $450 to “invest,” where to turn now?
4. Virtual Bank eMoney Market
Virtual Bank has a $20 referral bonus, much like ING, except the referrer gets $20 too and you can refer up to fifty people (that’s $1,000 in your pocket, contact me if you need a referral ). This offer is available to new accounts only, you’ll have to ACH money from an existing bank account, and the $20 isn’t available for the first 30 days. That’s all pretty standard online banking terms and conditions. So, after your account is approved you’ll immediately be eligible to start referring your friends. After you sign up, refer fifty people and you’ll have $1470 to invest.
Now that you have close to $1500, you should start moving towards conventional investments like investing in those stocks you always get emails about (just kidding!). Vanguard’s STAR fund with a minimum account balance of $1000 is an option but it’s up to you what you’d like to do. You can always peel off some of that money and go back into doing bonuses but at least now you can read “How To Invest $1000” articles which are typically more exciting.