It is a port multiplier, some kind of a HUB allowing to plug 5 Serial ATA drives on a single connector.
This product is sold $85 in the US as an electronic card that has to be installed in a case, which is quite easy.
This multiplier performances are quite good, even if you can't expect a miracle if you make a massive RAID, knowing that all date will transit through a single SATA channel anyway.
AMUG's tests confirm there's a problem with the latest PCI-Express G5s. The SATA transfers either via the integrated ports or an additional card are still way behind those of ancient PCI-X G5s. The output of those is barely 10% superior.
[update]: Adam Levine has posted some tests of a 8 Drive SATA II RAID0 system using a PMG5 PCIe equipped with a Sonnet e4P and aPort Multiplier: http://www.squeezeanickel.com/SATAII/
