Grandma's Unsulphured and Plantation Blackstrap (Unsulphured) are the two most popular brands. I couldn't name a third.
The Plantation brand seems to be more expensive but that might vary by location. I've always used Grandma's at a rate of 1-2 tbsp per gallon. You aren't likely to harm the plant by giving it every watering, but I am sure there is a point at which the plant nor the microorganisms in the medium can process/uptake and you'll probably be wasting after so much.
Oh yes, and there is things like ants to consider if those are a problem for you.