Nmap

Nmap is the defacto standard port / network scanner. It is good.

There is a book available for nmap. I think there's a mostly complete pdf available online.

Ping Lan Address Range and Report who is Active

$ nmap -sP 192.168.1.0/24

Get DHCP Servers in LAN

$ sudo nmap --script broadcast-dhcp-discover -e eth0

Basic searches

The second thing to remember with nmap and searches. -T is tcp. -U is udp. -P is a ping (check that host is alive) test. So remembering these three simple switches, gives you the following commands

$ nmap -sP          192.168.1.0/24
$ nmap -sT -p1-1000 192.168.1.0/24
$ nmap -sU -p1-1000 192.168.1.0/24

port / service detection (searches first 1000 ports, can take minute or two)

$ nmap -sV ipaddress 

man page has other info

See also tcpdump.