Value of last populated cell in column

Depends really... if the data types are constant... ie if always same datatype:

TEXT:
=LOOKUP(REPT("z",255),C:C)

NUMBER:
=LOOKUP(9.99999999999999E+307,C:C)

LOGICAL:
=LOOKUP(TRUE,C:C)

If it's varied and depending on if you have Blanks interspersed in your range or not:

NO BLANKS INTERSPERSED IN RANGE
=INDEX(C:C,COUNTA(C:C))

YES BLANKS
=LOOKUP(2,1/(C:C<>""),C:C)