Solaris

$ /usr/sbin/prtconf |grep -i "Memory size"
$ swap -s
$ df -k
$ /usr/local/bin/top
$ vmstat 5 100
$ sar -u 2 100
$ iostat -D 2 100
$ mpstat 5 100

#Solaris: Total Memory Size:
/usr/sbin/prtconf 2>&1 | grep -i "Memory size" | awk '{print $3" mb"}'
#Solaris: Free Memory size:
vmstat 2 2 | tail -1 | awk '{print $5/1024" mb"}'

ps -ef | grep smon | grep -v grep

HP-UX 11.0

$ grep Physical /var/adm/syslog/syslog.log
$ df -k
$ sar -w 2 100 
$ sar -u 2 100
$ /bin/top
$ vmstat -n 5 100
$ iostat 2 100
$ top

AIX

$ /usr/sbin/lsattr -E -l sys0 -a realmem
$ /usr/sbin/lsps -s
$ vmstat 5 100
$ iostat 2 100
$ /usr/local/bin/top # May not be installed by default in the server

Linux [RedHat 7.1 and RedHat AS 2.1]

$ dmesg | grep Memory
$ vmstat 5 100
$ /usr/bin/top

gnome-system-monitor & 

References: