Capital One 360!
They used to be ING.com which is when I started with them in 2005. They were taken over by Capital One a few years later.
As long as you're able to link your savings account to another bank to transfer $ in and out you're good.
There are no fees unless you're overdrawn and no minimum balances. Once you're a customer, you can have up to 25 savings accounts with them.
So I have one for Emergency Fund, Car Insurance, quarterly sewer bill, Vacation, College Funds, etc.
They also offer free checking. Again, no fees or minimums as long as you're not overdrawn. Free ATM. This is what I used as a separate checking account when I first started to pay my weekly snowball payment so it didn't blend with our regular checking account. I treat it like it's just another bill.
In the last 2.5 years I've gotten better/stronger with snowballing and now transferring to another account only delays making the payment another 2 days later. Now I just schedule the snowball payment to occur the day before payday and then on payday the money goes in and the payment comes out. Almost instant gratification.