Formal:
* Impecunious
* Indigent
* Destitute
* Bankrupt
* Insolvent
Informal:
* Broke
* Flat broke
* Skint
* Without a dime
* Rags to riches (this one is more of a phrase describing someone who was once penniless but is now wealthy)
The best synonym to use will depend on the context and the desired tone.