nmap

scan for IPs and ports
nmap -PR -oN /tmp/nmap.out 192.168.1.0/24

search hosts
nmap -sP 192.168.1.*
nmap -PR -oN /tmp/nmap.out 192.168.1.0/24

scan for open ports
nmap -T4 -sS 192.168.1.100

get more detailed information
sudo nmap -O 192.168.1.100 -T4

scan one port
nmap -sV -P0 -p 22 -vv 192.168.1.100

LINKS
http://linuxwiki.de/nmap