The whole program in general...I've even seen comments by really knowledgeable producers on here that have overlooked a lot of its 'hidden' features. Anyway, here are some of my personal favorites...
FL Scratcher - One of the dopest scratch programs out. You have several options with it, you can program your scratches by hand, you can use your mouse to control it, you can use your keyboard, or you can use turntable controllers.
Gross Beat - Is mainly used for Trance/House/Techno type effects but also has some turntable style scratches. Gross Beat is all about programming your scratches but it has some dope presets as well.
Fruity Parametric EQ2 - Is basically a 7 band EQ with a analyzer in the backgraound. This really comes in handy when you want to cut specific freqs. out of a sample. If you set it up right, you can come very close to eliminating all the vocals or all the bass, or even all the music out of a sample.
Fruity Stereo Enhancer - Is awesome for putting a stereo effect on anything you run through it. I use it a lot on my snare and hihat hits. It's really good for throwing a stereo offset on whatever.
Fruity Love Philter - Is dope for adding that phase shifting effect to any of your work.
Fruity Chorus - putting a chorus effect on vocals or any instrument. Really dope!
SliceX - Even I slept on this until recently. Edison is awesome alone but SliceX is Edison on steriods! Combine SliceX with your FPC and now you're playing with power. The chop dump function is soooo much easier in SliceX if you're running a pad controller it's ridiculous.
Drumaxx - Highly slept on! If you like creating and programming your own drums, it doesn't get much better than Drumaxx. With Drumaxx, you can go into the finest detail of any drum that you use. You can change the amplitude, decay, noise and noise phase of your mallet. You can change the tension, material, size, shape and other aspects of your membrane...let me put it like this...if you dig into drumaxx, you'll find how dope it really is.
Fruity Reeverb 2 - One of the best on the market and I'm a vst dude, trust me...there are some better but Reeverb 2 can run with the best of them.
Also, on a side note, FL Studio has a very cool way of side chaining your kicks (if you're into that). I've only found the need to do it once but there's a good vid on youtube that explains how to do it.
There are tons more but I'm gonna leave room for you guys to add, these are just some of my favs.
Just keep in mind that with all of these...you'll find some really good presets but if you dig deeper and learn the settings, you can come up with some awesome results. Most of these have some very good youtube tutorials as well, check the Image Line website too.