FL Studio has an extensive line-up of plug-ins, and some of our most oft-used of the existing lot are Maximus (a multiband compressor/limiter), Parametric EQ 2 (sonically transparent and visually stunning), SliceX (a wave editor, slicer and player all in one) and the stonking Sytrus synth.
We've also become big fans of Fruity Convolver, a convolution plug-in that's great for reverb and sound design, and the physical-modelling drum synth Drumpad - both were added since our FL Studio 9 review with the v9.1 update.
Slightly misleading is that demo versions of the Newtone and Pitcher plug-ins are claimed as new features and even appear in Image-Line's promo video. However, the full versions aren't included with any version of FL Studio and are in fact separate add-ons.
As for included new plug-ins, there's Patcher and ZGameEditor Visualizer, which creates trippy demoscene-esque visuals based on audio input. Parameters can be automated to make the visuals fit your tune and you can render the lot out to video. There are already some impressive user-created videos on YouTube that showcase the potential of this intriguing device.
One of the best new additions in FL Studio 10 is the Patcher plug-in. This is a modular chaining environment that enables you to load instruments and effects into it and freely connect them in all manner of configurations.
Crazy chains are easily achieved since you can have the audio flow into separate branches that converge and split multiple times. You could make a custom multiband effect by branching an audio source into multiple EQs - one for high frequencies, one for mids and one for lows - then sending the signal from each EQ to its own effect plug-in.
In our tests, Patcher became very buggy when we were working with third-party VSTs. For example, we experienced graphical glitches when trying to turn the knobs; we had FL Studio crash completely, plus one graphical freak-out; and linking knobs didn't always work.