SuperMicro sbi-7226t-t2 TwinBlade

Now that we've discussed the chassis, let's talk about the blades themselves.  SuperMicro has blades optimized for storage, for GPU usage, or simply for traditional builds.  The TwinBlades are optimized for density, packing two entire dual-socket systems onto one board.  It's actually quite similar to their "1U Twin", but with both systems on a single PCB.
 

Each of these systems gets two LGA 1366 sockets, 8 DIMM sockets, 2 x 2.5" HDD hot-swap bays, and dual gigabit ethernet ports.  They also have the option of a 10GBE or Infiniband riser card.  They also have onboard IPMI, which coupled with the chassis's CMM, gives you a very powerful remote management capability.  Individual systems (systems, not blades) can be powered up, powered down, reset, have remote KVM, be monitored for temperature and electrical draw, and more.   CPUs may be used with a TDP up to 95W.