Find the last row with a value.

Say I have a worksheet with a data entery range of A1 to A100. The user is
required to enter data from row 1 downwards and cannot leave blanks between
rows.

I want to return the value in the last comleted row by function. Any ideas?
For example that last row with a value may be in cell A20 of the A1:A100
range.

Rgds,

Bruce