First up HR. You I dont think you would ever manage to sell this as a web app. Personal employee details are, well, sacrosant property at any large company. Trusting such details to a web startup is unlikely to happen.
Handytree - Im not sure I get this idea. There are a few decision making startups kicking about (hunch for a start) you might want to research. I think pitching this to people as a site to use would be harder than most.
Mathtribes. This to me seems your strongest idea - but also a hard one to make money off. Math education is usually well laid out in the National Curriculum's of individual countries so you have to make sure you focus any lesson plan material to get it right (if you dont cover the curriculum exactly Teachers are unlikely to use it). I like the Q/A methodology idea though - that could be handy. But as I said - how to monetize it is a problem.