ING Direct is the granddaddy of this type of online savings account. It has been offering its no-minimum Orange Savings account for years.
EmigrantDirect just started offering its AmericanDream Savings Account around the start of the year. Its 3.00% APY was successful in getting attention. At this time, ING's savings account was only at 2.35%.
Then came HSBC's Online Savings account in March. For the first few months, HSBC's savings account offered rates comparable to ING.
Throughout the year, EmigrantDirect has maintained a lead over the other two. At the start of the year, EmigrantDirect was 0.65% above ING. But ING has been consistent in raising rates. Currently, ING is only 0.20% under EmigrantDirect. In the last two months, HSBC has gone past ING, and as of yesterday has even gone past EmigrantDirect.
Note, I had to review several sources to retreive this history of interest rates. I might have missed a few changes. Also, some of the dates may be a little off. Feel free to leave a comment if there's something that I missed.