depends on what your trading methodology is. i get some alerts from RT's hottrend scanner. i get some alerts from ISLAND's 'most up' / 'most down' system (and then look at L2/intraday charts). you probably want to determine what your system is - do you buy breakouts / sell breakdowns? sell tops / buy knives? buy pullbacks / sell pullbacks? before determining the type of scanner you'd need.
I'm testing out Trade Prospector for the 2 week trial. Downloaded this weekend, it looks promising. I'll let you know how it goes.