Money market mutual funds are a viable option if you want to invest extra cash in a relatively safe, low-risk deposit with liquidity. Through money market funds, your money is invested in high-quality debt for the short term. So, your money is comparatively safer than in mutual funds. 

The best money market funds today offer decent expense ratios, a fair yield and are low risk as they invest in government-backed securities.

Invesco Premier Portfolio Fund (IMRXX)
With a minimum buy-in of just USD 1000, the IMRXX is an accessible fund for many investors. It primarily invests in short-term instruments issued by banks and other financial corporations with top credit ratings. It also partly invests in U.S. government-issued debt securities. 

Fidelity Money Market Fund (SPRXX)
If you are a regular investor looking for a fund that has low to zero minimum investment, then the Fidelity Money Market Fund is for you. The fund has a varied portfolio, with a quarter of its assets invested in financial services companies’ instruments. It also invests in domestic and foreign-issued money-market securities, governmental securities, and repurchase agreements.  

Vanguard Federal Money Market Fund (VMFXX)
Launched in 1981, the Vanguard Federal Money Market Fund (VMFXX) is a conservative government money market fund that aims for the preservation of capital and earning of current income. The fund mainly invests in high-quality, short-term government securities. So, it is an ideal choice for risk-averse investors. U.S Treasury Bills, government obligations, and repurchase agreements make up a large chunk of the allocation. 

Wells Fargo Cash Investment Money Market Fund — WFAXX
Here is a money market fund with a broad portfolio. It consists of eligible money market investments that include bank obligations, corporate-issued instruments, asset-backed securities, government debt, banknotes, and more. It has a minimum deposit of USD 1,00,000 and aims to preserve capital and earn a high current income. 

Fidelity Government Money Market Fund (SPAXX)
As of May 2021, the Fidelity Government Money Market Fund (SPAXX) is one of the most popular in its category. Much like its other counterparts, this fund also focuses on capital preservation, high returns, and high liquidity. Since it is a government fund, most of it is invested in short-term U.S. government securities. 

With these funds, you can not only grow your wealth but also park your money safely without taking much risk.