Insider Buying
  
See: Insider. That's who's doin' the buyin'.
Insiders...think: people with way more knowledge as to the prospects of the company than the normal investing public. That'd be people like the CEO, the CFO, board members, the founder. When they're buying the stock (and they have to publicly disclose that they intend to buy the stock first, how much they'll buy, at what price ranges they'll buy, and a bunch of other strictures)...pay attention. If someone knows a lot more about something than you do, it's probably worth at least a nod of notice.
See: 10b5-1.