As far as the 'Big 3' or top contenders online, I comprised this list, accurate as of this morning:
(Ranked by APY)
GMAC 4.75%
CAPITAL ONE 4.55%
EMIGRANT DIRECT 4.50%
HSBC 4.50%
CITI 4.50%
ING DIRECT 4.15%
It should be noted that GMAC does state that there are fees for account balances under $500.
From the 'benefits' section: "No monthly service fees when you maintain a $500 minimum balance ... How does it work? All it takes is a $50 minimum opening deposit, and there’s no minimum to maintain. Balances under $500 are subject to a monthly maintenance fee." I did not investigate the others but I think GMAC is the only one which has this requirement.
Emigrant and some of the others make it more difficult to add more than one account to the externals. It must be done by snail mail. Capital One, for one, allows you to set up connectivity to two external accounts online.
Also, the Citi online savings apparently requires you to have a checking account with them as well, so if you aren't a checking customer already, opening a savings comes with a free checking account.
So, there's some info for people considering opening or moving their online savings.
Also, don't forget the SavingAdvice offers:
ING Direct bonus
Emigrant Direct unofficial bonus