Prove Subset-Sum is NP-complete - Alternative reduction?
In general, each $NP$-hard problem can be reduced to all other known $NP$-hard problems (there exist hundreds).

For the subset-sum there is for a example knapsack. Subsetsum is a specialisation of knapsack.

The other way, if you could solve subset-sum in poly-time, then you would also be able to solve PARTITION (and 3-PARTITION and 4-PARTITION)