HNFO writes: "HP is unveiling their new 'blade' servers that fit onto a single card. Their press release is here. They are currently available with your choice of RedHat, Debian and SuSE. A picture of the card can be found here and a picture of the chassis can be found here."
If you're looking for high-density slot-based computers, earlier postings about RLX's Transmeta blades
OmniCluster's x86 variety
might interest you as well.