In no particular order:
NI Absynth 5 - Very handy for creating pads and soundscapes although it doesn't stop there, it's kinda modular and can do multiple types of synthesis. It also has multiple filter modes including some unique ones such as Supercomb and Cloud. The effects are also pretty sweet.
U-He Tyrell N6 (FREE) - Very nice sounding free synth. It doesn't have a whole lot of features but I've gotten some very warm sounds out of it. One of my favorite features on Tyrell is the glide offset for OSC2, which can be really handy and render cool effects!
NI Guitar Rig - Pretty much an effects rack with a Reason-esque feel. Some of the add on content you can get for it is pretty rad, such as Traktor's 12 and Rammfire. Obviously it's good for guitar, but it works well on other stuff too.
NI Razor - You can get some crazy sounds out of this guy! It has a very unique take on additive synthesis and you'll find something unique in every section (oscillators, filters, fx). Especially useful if you want to make some nasty sounds. My only complaint is that porto/glide sounds very samey on most things and has a particular sound that I'm not really fond of.
Bram's Supatrigga (FREE) - Feed it audio in and it randomly spits out your audio rearranged, repeated, reversed, and slowed down. It's very useful for coming up with ideas and interesting things you wouldn't have thought of. It doesn't have to do all those things at once though, and sounds wicked if you put it on some drums or a pad that has a lot of movement. Also, it can really twist up your sound when combined with DFX Buffer Override.
Sugar Bytes Artillery - Really useful plug if you like to mash up your beats with effects tied to real world controls. Basically you select a key (or group of keys) on the keyboard and assign an effect to it. You can do this across the entire keyboard if you'd like, then press play and mash up your sound! The effects all have parameters that you can fiddle with to adjust depth, rate, frequency - based on the key you press. I find the Looper to be the most helpful. You can get a free version in Computer Music magazine.
U-He Zebralette (FREE) - This is a free version that is supposed to be a single oscillator from Zebra 2, don't let yourself be fooled though, it's capable of a lot! It can morph between different slides, where each side can have a different waveform that can be drawn on it, so it's kinda like a wavetable synth a little bit. It also comes with 2 separate effects banks, one that has the usual modulation effects (chorus, phaser) and a multimode delay, and the Spectral effects section which are all pretty unique and can radically change your sound - especially when their depth is tied to an LFO or envelope.
Audio Damage Roughrider (FREE) - This is a brutal compressor. There is also a Pro version which I think is more of a multipressor and you obviously would have to pay for but this one works nice too. The dials don't have any numbers on them and theres no pop up that tells you the value you're on so you really have to use your ears to find the right sound.
Synapse Audio Plucked String - The only way I know to get this one is through a copy of Computer Music magazine but it's a really nice sounding plug. Theres not a whole lot of features but I think it uses modal synthesis to make its sounds. It sounds really great and obviously works great for plucks. One of the great things about it is that you can adjust the levels of the string and pluck individually.
Togu Audio Line (FREE) - Everything from TAL is fantastic, for being free that is. The top of the pile is clearly Noizemaker which, imo, has a fantastically dirty saw wave and the filter sounds really nice. I regularly draw for other TAL plugs too, such as Tube, Filter2 (envelope filter), and Dub3. I think every producer should have at least one TAL product in their arsenal.
Honorable Mention: Lowcoders King Dubby (FREE) - Just about any interval you'd want for BPM synced delay, complete with filter degradation.
