Vanguard has several money market funds, with varying degrees of risk:

• Tax-Exempt (highest yield): Muni gov't only
• Prime: Mostly AAA (gov't, CDs, commercial paper, and other stuff), with high quality and low expenses (so, no need to boost return with risky investments)
• Federal: Gov't and gov't-backed
• Treasury (lowest yield): Gov't only

For some weird reason, the market has decided that muni gov't is the highest risk of all of these. So, you have a range of money market funds, but all of them are low (not zero) risk, IMO. I can't speak to Fidelity, as I'm not familiar with those.

Dan