General:
* Shortly
* Presently
* Directly
* In a little while
* Before long
* In due time
* Shortly
* Imminently
* In the near future
More formal:
* Forthwith
* Immediately
* At once
* Without delay
More informal:
* Any minute
* Right away
* In a jiffy
* Any time now
Emphasizing time:
* Very soon
* Quite soon
* Soonest
The best synonym depends on the specific meaning you want to convey.