I assume that you are asking about actions that were not exactly neutral but which fell short of actual acts of war. If so, the major things that the US did included:
- The destroyers for bases deal that FDR made with Britain. This gave Britain ships and it freed up British forces (from the bases) to go and fight.
- Lend-Lease. While still maintaining neutrality, the US sent huge amounts of war materiel to the British and the Soviets.
- Having the US Navy protect convoys of ships (both American and British) going from the US to Britain.
All of these actions were clearly intended to help the Allies win the war even though the US was still officially a neutral power.