OpenNebula

onevnet should have an option where you can request the first free IP address in a VNET / AR.
Sunstone could also use this feature (onevnet getip $vnet) in the TEMPLATE / VM (Attach new nic) dialogs to automatically fill in the first free ip-address in the "IP field"
Use case: useful in scripting: no need to do all kind of sorts / awks /seds / greps to figure out the first free ip of a VNET / AR.

onevnet hold should have an option to hold the first free ip-address
Use case: Not only do you get the first free ip, you know it's hold and you can release it just before you instantiate / release / deploy a template

onevnet reserve should have an option to make a vnet reservation of the first free ip-address.
Use case: get an ip know, think later. If a template needs to be created at some point in time, but it needs a static ip address you want to know for sure is availble. Ideally you could add an option to hold the ip as well and make it an atomic "reserve and hold" operation.