Open-Architecture hardware round-up...
First up, we have the Chameleon, from Soundart. Unfortunately, Soundart went out of business not too long ago, but I'm given to understand they still have some available, for around $550 when all is said and done. I've seen these on the 'bay as well now and again, so they're not unobtainable. I own one, as does Adam, and while I like mine more than he likes his, we both like it just fine. Luckily for us, Paul Maddox (of Monowave fame) has kind of taken the Chameleon under his wing, and made a site devoted to furthering software development for it, which you can find here. All these "skins" are in addition to the several that you can get on the Soundart site, so all in all, this particular unit probably has more good quality programs available for it than any other option, cost-wise.
Now, by Open-Architecture, I want to be clear that I don't mean "open" like Open Source. I mean open like it isn't a chunk of DSP that is permanently stuck being, e.g., a Virus. In that vein, next up is the Roland VariOS. This particular beastie is significantly more powerful than the Chameleon, but only has three "skins:" VariOS-8, which is a Jupiter 8 emulator, VariOS-303, which does what you'd expect, and V-Producer, which is a slightly dumbed-down version of the VariPhrase box they used to hype the shit out of. You can read about it at the Roland site here.
This unit is also discontinued, I think, but it is available here and there as Roland sells down their stock. Sweetwater has it for $845. Worth it? That's up to you to decide.
Now, the Nord G2 Engine, well, you know all about it. It's a Nord G2 with no front panel; all programming is done via the USB interface. This isn't open in the strictest sense, as it only runs one program, the G2 Modular stuff. But it is certainly as programmable as the other two options, and considerably moreso in the case of the Roland unit. It is also much more popular and is still being made. It'll set you back a grand, if you shop around. I think that everything Nord makes kind of has this Nord-ness to it, and my experience playing with the G2 demo doesn't really change that. But if you dig the Nord, you could do worse than this.
Last up, we have the venerable PowerCore FW. This isn't as adaptable as the other three, as it is just a brick of DSP that needs a DAW to function. However, when it comes to professional-quality programs, it beats the other three hands-down. If you end up getting the really good plugs, this will end up setting you back an arm and a leg, but it's the best game in town for open systems, no doubt about it.
The point to all of this? If you're going to be buying a DSP-based synth or effect any damned ways, you might as well get something that can be more than one thing, you know? The current crop of Virtual Analogs and multi-effects are all intrinsically the same thing inside; it's the programs that count. So shop for the best programming options, not the best looking front panel.