You can lookup the IP address by hostname. Assuming the hostname of your Pi is "raspberrypi"
Here is how you obtain the IP address using ping command and hostname.
From Linux: ping raspberrypi
From Windows: ping raspberrypi
From OS X: ping raspberrypi.local
In order for the command to work from Windows, you need to have Samba running on your Pi. You can install Samba using the command
sudo apt-get install samba