General:
* Substitute
* Successor
* Stand-in
* Backup
* Alternate
* Reserve
* Proxy
* Surrogate
More specific:
* For a person: Understudy, heir, inheritor
* For a thing: Spare, spare part, duplicate, copy
More informal:
* Fill-in
* Stand-in
* Takeover
The best synonym will depend on the specific situation.