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

# TCP
nmap -T4 -sS <HOST_IP>
 
# UDP
nmap -T4 -sU <HOST_IP>

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