As you may know, if you install raspbian on your Raspberry Pi and connect it to the network, the system will boot up and will try to get an IP via DHCP. Then you can access to you Raspberry using SSH (is enabled by default) but, how do you get the IP of the Raspberry the first time?

The easiest way of doing it is  using NMAP:

$ sudo nmap -sP 192.168.0.1-255

inedit@tpad:~$ sudo nmap -sP 192.168.0.1-255  
Starting Nmap 6.00 ( http://nmap.org ) at 2013-01-27 20:40 CET  
Nmap scan report for 192.168.0.1  
Host is up (0.013s latency).  
MAC Address: 64:16:F0:XX:XX:XX (Shehzhen Huawei Communication
Technologies Co.)  
Nmap scan report for 192.168.0.10  
Host is up (0.013s latency).  
MAC Address: 8C:A9:82:XX:XX:XX (Intel Corporate)  
Nmap scan report for 192.168.0.11  
Host is up.  
Nmap scan report for sam (192.168.0.16)  
Host is up (0.012s latency).  
MAC Address: B8:27:EB:XX:XX:XX (Raspberry Pi Foundation)  
Nmap done: 255 IP addresses (4 hosts up) scanned in 2.28 seconds

As you can see, three hosts has been detected and the last one is a Raspberry Pi Foundation. NMAP knows the vendor using the first 3 bytes of the MAC address, called OUI (Organizationally  Unique Identifier). In this example the Raspberry Pi IP address is 192.168.0.16.

When running the command ensure to do it in the same network. Example: 192.168.1.1-255 or 10.1.2.1-255 or .....

Update: There is other way of doing the same thing. Check it out!