Problem:

Column A contains a list of numbers that is being continually added to at its base.
We want to create a formula that will retrieve the last value in the column and automatically update when a new number is added.

Solution:

Use the OFFSET and COUNTA functions as shown in the following formula:
=OFFSET(A1,COUNTA($A:$A)-1,0)

Note:
The formula will only work correctly when there are no empty cells within the list.