Money Market Funds
You can consider Money Market Funds the lowest risk investment around, though there is a bit of risk as they are not FDIC-insured. The interest rates are not guaranteed so you need a little more hands-on approach when dealing with this type of savings account.
Money Market Deposit Accounts
The big difference between these type of savings accounts and general Money Market Funds is that Money Market Deposit Accounts are protected by the FDIC. The savings does require some stipulations like you must make some type of minimum initial deposit as well as maintain a certain balance at all times. You will need to speak with the individual bank or credit union for more details regarding a Money Market Deposit Account
Treasury Bills & Notes
When you save with a treasury bill or note you are investing with the U.S. government. Treasuries have several advantages such as being tax exempt, and available in many different maturity lengths. The bills are sold at varying discounts and then worth their full face value once it matures. So, for example, paying outright for a $470 bill that is worth $500, would return $30 in investment once the treasury matures. The biggest drawback to treasuries is they take awhile to mature, often two or more years, but they also collect fixed-rate interest at the same time as well.
Bonds are similar to treasury bills and notes, you are investing in the U.S. government and granting them funds for projects. They reward you in return for your investment with fixed interest.