A pretty cool (free) plugin would be the TAL Noisemaker. As with most free VSTs, it sounds a bit cheesy, but it's better than what you normally have to work with. For some organs/weirdish piano sounds, try Lazy Snake.
But there's plenty of good VSTs included with FL Studio (I have the demo, though, not sure if they're in other versions or if you have to buy them separately). There's FL Keys (or is it fruity keys?) for piano, and Sytrus is cool for techno-y things. Don't know of any other ones, if anyone else knows, please share. I'm always looking for new VSTs too, specifically one with trance sounds.