You can do a ping sweep with nmap. Nmap has a gui, or you can run it command line. Here is a sweep of my home network.
nmap -sP -PT 192.168.1.1/24
Starting Nmap 4.20 ( http://insecure.org ) at 2008-03-18 11:41 EDT
Host 192.168.1.1 appears to be up.
Host presto_2 (192.168.1.15) appears to be up.
Host presto (192.168.1.22) appears to be up.
Host duelie (192.168.1.26) appears to be up.
Nmap finished: 256 IP addresses (4 hosts up) scanned in 2.232 seconds
BTW, I'm behind a firewall, using NAT.
Hope this helps.