Vanguard and Fidelity 的money market funds收費最低

要是買券商的money market funds, expense ratio would be around 0.4%.

There are two types of money market funds in general:

1, invest only in government instruments 

2, invest in both government and corporations instruments

In general, money market funds are low risk. But after 2008, people realize that they also carry high risk if Financial market exploded. So T-bills are more popular to park cash.