Here are some possibilities depending on the context:
If you're talking about someone being used for their skills or abilities:
* Resource
* Asset
* Workforce
* Employee
* Talent
* Instrument (implies being used to achieve a specific goal)
If you're talking about someone being exploited or manipulated:
* Pawn
* Puppet
* Tool (the word itself, but with a negative connotation)
* Gofer
* Stooge
If you're talking about someone being helpful or supportive:
* Helper
* Assistant
* Collaborator
* Teammate
It would be helpful to provide more context to get a more accurate synonym.