AIX:
lsconf
ll /var/adm/ras/bosinstlog

ESXi:
get-vmhost | sort name | foreach {$h = $_.Name; $d = (Get-EsxCli -VMHost $h).software.vib.list() | sort InstallDate | select -first 1; write-host $h,$d.InstallDate;}
HP-UX:
ll /dev/config
echo "sel dev 1;info;wait;il"|/usr/sbin/cstm

Linux:
rpm -qi basesystem
rpm -qi filesystem
head /var/log/installer/syslog

Solaris:
showrev
ll /var/sadm/system/logs
/usr/sbin/prtconf | head

Windows:
systeminfo | findstr Original