I keep 5k at Mango Money earning 6%.
It's been stated a few times but HELOCs and credit cards aren't good emergency funds. Some people had them closed down in 2008, but what was more common was to have the limit drastically decreased to the point they were basically frozen.
They can and may still do this, in response to a major charge.
My mom was basically ruined by using her HELOC, not even anywhere near the limit. Her loan then got sold to a sleazy subprime operator (Ocwen), who keep overcharging for her escrow account, misapplying payments and then charging BS fees, etc.
Credit card companies do this as well, in fact it's very much their MO. Make a big charge and they'll lower your limits, raising your utilization even more, purposely lowering your credit score so they can both charge you more interest and fees, and kill your options for balance transfer. You can usually avoid this by paying in full within 30 days, so you're probably OK if you have enough elsewhere to cover it.