I'm in the very conservative VMMXX money market, and have been happy with it. It's a hold steady, preserve investment, pay small dividend money market fund.
Since we're FIRE, everything feels sort of like an emergency fund, but right now, I keep the equivalent of one year's worth of income there. We have a few hundred thousand in cash right now, but need to get around to moving more of it into this. We're "only" 14 months into FIRE and I've still got administrative stuff like this that I need to take care of. :)
Before I was FIRE, I kept six months worth of emergency money stashed (enough to cover all living expenses with a budget of $300/month for minimal groceries) and just added more over time. It grew over time: when I had almost no money to my name, it was $500... then $1,000... and so on.