I have used RealTick's API. MyTrack (TrackData) also offers an API. I hear eSignal will soon offer an ActiveX control to pull data through their client app.
Of those three, MyTrack is probably the cheapest. I hear they have good data, but I haven't used them and can't vouch for it.
Come to think of it, I think DTN.IQ also offers an API for IQFeed.