I invest in the Municipal Money Market Fund which has been among the highest rankings for many years. The current rates actually beat many of the taxable savings account rates. Even the Vanguard Prime Money Market account rate barely beats the Municipal Money Market account rate and you have the added tax exemption bonus. I used to invest most of the money in the US Treasury Vanguard Fund, but rates there are even lower than most online banks. You have checkwriting privileges and can request electronic transfers to your external bank accounts.