Lake trout, brown trout, largemouth, salmon, suckers. Supposedly muskie run there in the summer, but I've yet to see or catch one there.
I just started fishing it maybe six months ago and have probably been out a dozen times. I did some dock fishing growing up, but the first time I pulled a big fish out of the lower niagara my mind just about exploded at the size and power. I'm always surprised there aren't more people fishing it- I bet it's some of the better freshwater fishing there is. Again though, the stairs and the wicked current may be prohibitive to some.