Address last numeric cell:
=CELL("address",INDEX(1:1,MATCH(9.99E+307,1:1)))
Address last text cell:
=CELL("address",INDEX(1:1,MATCH(REPT("z",255),1:1)))
Address of the last non-empty cell (with any value), control+shift+enter, not just enter:
=CELL("address",INDEX(1:1,MATCH(9.99E+307,1/(1-ISBLANK(1:1)))))