So far, the best deal I've found is ING. I'm up to a 4.5% interest rate with no fees or minimum balances, and I can access the money any time with an electronic transfer. This is something I use to supplement what I keep in checking - in case of car repairs or other bills coming up, but I have a few days headway. The interest rate is great for this.
Online banks can offer great rates for such a purpose. I keep a fair amount in an online money market as well. It's a good place to keep cash reserves.