With my Raspberry Pi project, it has been using DHCP to get an IP address, and my network keeps assigning a different value (I know there are solutions to this but I haven't made use of them yet). So, I went ahead and installed nmap using the homebrew package manager for OS X:
$ brew install nmap
And now, I can scan the local network to see what devices are up:
$ sudo nmap -sP 192.168.1.1-254 Starting Nmap 6.00 ( http://nmap.org ) at 2012-06-10 10:12 EDT Nmap scan report for 192.168.1.1 Host is up (0.0041s latency). MAC Address: B8:8D:12:5A:00:00 (Apple) Nmap scan report for 192.168.1.2 Host is up (0.0039s latency). MAC Address: E8:E0:B7:17:00:00 (Toshiba) Nmap scan report for 192.168.1.6 Host is up (0.32s latency). MAC Address: B8:27:EB:4C:00:00 (Raspberry Pi Foundation) Nmap done: 254 IP addresses (3 hosts up) scanned in 5.46 second
The better solution would be to have the device use a static IP address (or even cooler, add a tiny LCD screen to show the current IP address :D).