Inventory scripts allow you to store your hosts, groups, and variables in any way you like. Examples include discovering inventory from EC2 or pulling it from Cobbler. These could also be used to interface with LDAP or database.
chmod +x an inventory plugin and either name it /etc/ansible/hosts or use ansible with -i to designate the path to the script. You might also need to copy a configuration file with the same name and/or set environment variables, the scripts or configuration files have more details.
Send in pull requests to add plugins of your own. The sky is the limit!