Can I take candid photos of people in publuc without their knowledge and post them on an ad supported blog or website?
It's not generally illegal to take photos of people in public and then post them on an ad supported website.
Just as it's not illegal for those people to sue the living daylights out of you for using their likeness to promote goods or services that they find offensive.
There are plenty of laws that can apply that are very fair in that regard - libel, passing off, etc.
It depends on the local law, that differs among countries. For example, in my country - Poland, it is legally allowed to take and publish photo containing a person as long as the photo is not a portrait-like - the person on the photo is not the main subject of it. But still there are few exceptions from this rule.