Back to the subject of third party softsynths for a moment...
I've invested in a ton of them, because why not? They're all different, each does its own thing, they're affordable, and so many are useful.
I agree, Zebra is wonderful, and the sound quality is improved in the new beta version. Check out Tone2's Gladiator; it is also a very interesting synth with an absolutely huge and unique sound. While programming may seem complex for a beginner, it has a truly useful tutorial, and learning it is a breeze.
I also like Cameleon, but find it's more useful as seasoning than main course, so to speak.
But the synth set that's been getting a lot of my time the past few weeks is the new DCAM set from FXpansion. If you want that analog sound, this is the closest I've found yet. The filters have that squelchy thing going, the envelopes are fast, and it's an interesting approach having three synths that can be separated or combined in the included Fusor multi-synth package. I've been comparing it with stalwarts like the Arturia models, and while Arturia sound great, they're slightly less real sounding. But this is a moving target, really, and it just keeps getting better year after year. DCAM has one caveat: you need a very fast multi core CPU.
I'd also like to recommend the Applied Acoustics modeling synths; again, each one occupies a certain sonic space, they are very low on CPU use, and they're very lovely sounding. For a long time, I've felt that these are truly under-appreciated. And you can do a TON of things with Tassman.
NI synths are classics, and the presets are very well programmed. The same goes for the LinPlug stuff. Octopus, which combines samples and FM in a way that reminds me of my old SY99, is particularly rich sounding. RMV is a bit strange but the loops and possibilties are really interesting. CronoX does really cool things as a sample based synth.
I've found the Rob Papen synths useful. The ideas are brilliant. My sole criticism is that at times they could use a little more "cut", and I think this has to do with the envelopes and filters. So, for example, while Albino is programming genius, I've got to use it with a compressor to fit into a mix unless I'm using it for a pad, etc.
If you like the vintage vibe a nice Mellotron can add, I haven't found anything that compares with SampleTron. And the Miroslav Philarmonik IK also sell was once a premier and very expensive orchestral library that STILL sounds great and I get a lot of use out of it. Other really nice inexpensive orchestral and piano libraries come from Garritan, and Kirk Hunter makes a version of all his really nice orchestral libraries for ESX, though I got mine with Independence Pro, which has incredibly well sampled sounds and is a very nice sampler that can do more than ESX.
One soft synth that intrigues me is Circle; you can do an awful lot with it, and I'm just working it into my working rotation. It's virtual analog, but everything can just about modulate everything else, and it's capable of some very cool sounds.
I hope this list sparks some investigation, there's so much good stuff out there!