Well, there's the first Connection Machine architecture, very roughly contemporaneous with Lisp Machines (I had to regretfully tell my friend Danny Hillis that LMI wouldn't be able to provide Lisp Machines for Thinking Machines Corporation in time (which had to be formed because the project needed 1-2 analog engineers, which MIT was structurally unable to pay, no one gets paid more than a professor). He was really, legitimately pissed off by what Symbolics did with Macsyma, a sleazy licensing deal to keep it out of everyone else's hands (and they tried to get everyone in the world who'd gotten a copy of it to relinquish it). Later neglected, even when it became the Symbolics cash cow.)
Anyway, if you're not talking ccNUMA, the limitations of which has got me looking hard at Barrelfish (http://www.barrelfish.org/), e.g. if you're talking stuff in the land of MPI, again it's going to be very hard to beat commodity CPUs.
Although in that dreaming, look at lowRISC: http://www.lowrisc.org/ looking at things now, they propose taping out production silicon as soon as 2016, and say 48 and 28nm processes look good. From the site:
What level of performance will it have?
To run Linux "well". The clock rate achieved will depend on the technology node and particular process selected. As a rough guide we would expect ~500-1GHz at 40nm and ~1.0-1.5GHz at 28nm.
Is volume fabrication feasible?
Yes. There are a number of routes open to us. Early production runs are likely to be done in batches of ~25 wafers. This would yield around 100-200K good chips per batch. We expect to produce packaged chips for less than $10 each.
And with a little quality time with Google, the numbers look good. Ignoring the minor detail of NRE like making masks, a single and very big wafer really doesn't cost all that much, like quite a bit less than $10K.
And we now have tools to organize these sorts of efforts, e.g. crowdsourcing. But it's not trivial, e.g. one of the things that makes this messy is modern chips have DRAM controllers, and that gets you heavily into analog land. But it's now conceivable, which hasn't been true for a very long time, say starting somewhere in the range between when the 68000 and 386 shipped in the '80s.