Command to get system hardware serial number.

SMBIOS/DMI standard includes system manufacturer, model name, serial number, BIOS version, asset tag as well as a lot of other details of varying level of interest and reliability depending on the manufacturer. This will often include usage status for the CPU sockets, expansion slots (e.g. AGP, PCI, ISA) and memory module slots, and the list of I/O ports (e.g. serial, parallel, USB).
Solaris X86:

$smbios
  ..
  Manufacturer: HP
  Product: ProLiant DL360 G3
  Serial Number: B038XXXXX
  ..

Solaris Sparc:
smbios is not supported in SPARC yet, the traditional command prtdiag works for both X86 and SPARC ,but it reports less detailed hardware information.

Linux/BSD:

$dmidecode